You continue to ask good questions. I divided my answers into sections for visibility.
As for replacing it with another fighting unit, it is viable if you accept -6 or more carryover gold loss because even the cheapest Elvish Fighter costs (-14-1)*0.4=-6, assuming the unit doesn't capture any extra villages. In 1.14, -6 carryover gold often prevents 63 carryover gold for 10 recruited/recalled units in S2 as explained in the walkthrough above, so I wouldn't recommend it. However, in 1.16~, it is a solid option for 2 reasons; 1. it is easier to earn more gold in S1 due to fewer enemy units (on average, 3 more carryover gold in my experience). 2. S2 is easier and playing with only 9 recruited/recalled units may be fine. As this example shows, you have a good point that replacing Elvish Scout with another fighting unit can be better in some cases.
XP for Elvish Scout is gained mostly in S3 with the help of S1~2 if the opportunity presents itself. As explained in the walkthrough of S3~4 above, 2 close-to-advancement Elvish Scouts are needed in S4 to make the water battle easier. S10 and 12 are also good places to gain XP because enough time is left after the village-grabbing job.![Very Happy :D]()
As for XP, it's true that Elvish Shaman requires much more careful management than Mage. It's often frustrating to calculate enemy HP just to give the last hit to Elvish Shaman. However, I see it as an investment with the mindset of "Mage is easier to use for now, but Elvish Shaman can flourish in the future."
S4: Recalled 2 close-to-advancement Elvish Scouts advance to Elvish Riders, and they join the water battle to help Merman Fighters.
S8: Some distances between 2 villages are 11. Only quick Elvish Rider (11 MP) is fast enough to move from one to another in 1 turn.
S10: Only quick Elvish Rider (11 MP) can reach the Mother Gryphon on turn 4 and then move to the eastern side to block enemy units on turn 5~.
S15: Just in case all Dwarvish Fighters are non-quick, recall an Elvish Rider/Outrider to save the ally leader.
I agree that it's always a challenge to provide enough XP to relatively weak units such as Elvish Shaman, Elvish Scout, Gryphon Rider, Thief, etc. I try to have a clear strategic goal in mind when I use these units. It's a pain to use weak units without a future plan.
"Elrian (Silver Mage) + A recruited one (White Mage/Mage of Light)" seems better than "Elrian (White Mage/Mage of Light) + A recruited one (Silver Mage).
Reason 1: Loyal units are often recalled on turn 1 to save gold, but healers can wait until turn 2~3 because healers are needed only after taking damage.
Reason 2: It's not necessary to have 2 healers in some scenarios. 2nd loyal healer can become redundant.
Reason 3: A quick healer (6 MP) can move 3 cave hexes in S15~17 while a non-quick one (5 MP) can only move 2 hexes, and Elrian is never quick. Therefore, when a Mage is recruited in S3, it makes sense to pray that it is a quick one, which has a 50% chance.
These are my reasons, but I'm not denying other playstyles.
Elvish Scout
As you pointed out, the Elvish Scout to the southwest in S1 barely makes its own cost back. As illustrated in the tables below, even in the ideal situation, a non-quick Elvish Scout actually loses some gold when the scenario is finished on turn 9. Nevertheless, finishing the scenario on turn 9 itself often guarantees enough carryover gold, so the Elvish Scout works as insurance for the possible turn 10 finish. Also, by recruiting more than 1 Elvish Scout, it is more likely to have the one with ideal traits. Because the difference between recall cost and recruit cost is only 2 for Elvish Scout, praying to roll the one with ideal traits in an early scenario and recalling it later is a valid strategy for Elvish Scout, unlike Elvish Fighter whose cost difference is as high as 6.Scout, Archer, and cost:
You are right. Elvish Scout is not a good combat unit, so using Elvish Archer should be the priority wherever possible. For example, in S3, Elvish Archer recruited at the southeast of Konrad on turn 1 is fast enough to capture 3 villages on turn 2~4 and join the battle afterward. However, in many other cases, Elvish Archer etc. is just too slow compared to Elvish Scout. As for the cost, 1 Elvish Scout probably should capture at least 3 villages to be net positive. For example, if it takes 7 turns until enough villages are captured to pay upkeep for all units, 1 Elvish Scout essentially costs 18+7=25 gold, which cannot be paid by 2 villages kept for 10 turns = 20 gold. In long scenarios, capturing 2 villages may be sufficient because more gold is earned per village, but in many cases, 3 seems the minimum. Also, I highly doubt that Elvish Archer "line" is S tier. Although level 1 Elvish Archer is decent in S1~4, level 2~3 archer units are pretty much useless because the opportunity cost is too high; XP is a limited resource, and XP in early scenarios should be gained to make White Mage, Elvish Sorceress/Enchantress, Paladin etc. who are more useful in difficult bottleneck scenarios such as S6 and 9. Early part of HttT has the campaign structure of “difficult scenario = the ones with skeletons,” which significantly reduces the importance of elvish/human archers.Oh I've no doubt that the Archer line isn't S tier. Like you said, I've also seen how Marksmen get severely eclipsed by the many magical ranged attackers that are available, and I also agree that the only time the Archer is useful as a L1 meat shield is in the very early scenarios.S Elvish Scout is the best scouting unit to take villages and earn gold. Sometimes, +5~10 gold in the next scenario makes it much easier.
But regarding your comment on the Scout's cost efficiency, I'd never thought about it that way, though of course it makes perfect sense. On S1 then the Scout that goes south actually barely makes its own cost back, considering how few turns there are. I also tend to have issues getting experience for Scouts when they're usually busy just running around, what scenarios do you usually find room for them to level?
Also, I guess this is shown by your experience, but is the "future investment" of the Scout always worth the opportunity cost in the immediate scenario? For example you get a net gain of a few gold from S1's Scout, but potentially lose out on experience that you might have gained from another Fighter or Archer.
Tables:

XP for Elvish Scout is gained mostly in S3 with the help of S1~2 if the opportunity presents itself. As explained in the walkthrough of S3~4 above, 2 close-to-advancement Elvish Scouts are needed in S4 to make the water battle easier. S10 and 12 are also good places to gain XP because enough time is left after the village-grabbing job.
Preparation
Yeah. Meticulous or sometimes nitpicky preparation was needed to overcome RNG. Gotta be ready for Delfador missing all 4 strikes (0.8%).So again it boils down to being extremely meticulous about planning for future scenarios, just like how you prioritize getting an Outrider purely to save the ally in S15, or training a Silver Mage primarily just for S9.Scenario 5a vs 5b:
- Thug
In short, the biggest reason to take 5b path is because 1~2 Highwayman is needed in S16. One important mindset I have in permadeath HttT is that protecting Li'sar consistently in S16 is one of the most, if not the most, difficult parts of the campaign. Most players probably don't care about it because they just complain about wesnoth AI behavior and restart the scenario when suicidal Li'sar dies. With my mindset of never complaining about AI or RNG, 4 strong melee units are needed by S16. While you are right in saying that Highwayman can be replaced by Dwarvish Lord, because Dwarvish Fighter joins in S15, only 2.5 Dwarvish Lords can be realistically made in a single scenario due to limited XP available. Therefore, Thug/Bandit is recalled in earlier scenarios to make 1.5 Highwayman by S16. In 1.16~, S15 is shorter, and less XP is available, so it is more likely that 2 Highwayman are now needed instead of just 1. In addition, Uncle Somf (loyal Bandit) in S13 tells Konrad which entrance is the right one, which is not available if 5a path is taken.
- Poacher
Poacher line has a very specific role in S10. Level 3 Ranger is fast and requires less MP in some rough terrains. Therefore, a quick Ranger (7 MP) can reach the Mother Gryphon on turn 5 before most enemy units arrive. It is not a must-have, but having it along with Elvish Sylph increases the chance of killing the Mother Gryphon in time.

Cannon fodder
That's right. In my 6 permadeath runs (total), 131 out of 140 Dwarvish Fighters are recruited/recalled only in 2 scenarios (S15 and 17). On the contrary, 117 out of 130 Elvish Fighters are recruited/recalled in 8 scenarios (S2~3, 6~9, 12~13).I'm surprised the Elvish Fighter got S while the Dwarvish one got A, along with the Elvish Archer. But I suppose there are many more scenarios where the Elvish one is the fodder of choice?S Elvish Fighter is the best cannon fodder. It is cheap, has high HP, and has both melee and ranged attacks.
A Dwarvish Fighter serves as the main force as well as cannon fodder in caves in S15 and 17. Making 2~3 Dwarvish Lords in S15 is a must.
A Dwarvish Guardsman is very good at holding the line with its steadfast ability.
A Elvish Archer is the primary ranged damage source but only in the first 4 scenarios.
Mage and Shaman lines
I highly value Elvish Sylph partially because I was speedrunning my 1.14 runs. Elvish Sylph enabled the assassination of the blue leader in S12, which saved as much as 1 hour in Run 3. Also, the assassination of Asheviere in S24 made it possible to finish the scenario in 10 minutes. That being said, even in non-speedrunning context, Elvish Sylph > Great Mage in my eyes. I can't emphasize enough the importance of slows weapon special. Also, without Elvish Sylph in S10, killing the Mother Gryphon becomes more RNG-dependent because Delfador and Kalenz can't necessarily reach there in time. Of course, nothing is wrong with playing with Great Mages, though.Is it that useful to build up an army of Sylphs rather than a few Great Mages? They do comparable damage and although I see the power of Sylphs and Shamans, I've still found them more challenging to level up than Mages due to their weak attack.B Mage has a good magical ranged attack, but only 1~2 recruits are needed due to loyal Elrian and Moremirmu (White Mage).
As for XP, it's true that Elvish Shaman requires much more careful management than Mage. It's often frustrating to calculate enemy HP just to give the last hit to Elvish Shaman. However, I see it as an investment with the mindset of "Mage is easier to use for now, but Elvish Shaman can flourish in the future."
Horseman and Elvish Rider
Yes. I didn't really use Horseman after S6, but S23 sounds like a good place because 33~50% of enemy units are Bowman with relatively weak melee retaliation.I'm guessing this is because there are relatively few scenarios where cavalry are useful? I often also use them in later scenarios such as S23.B Horseman is good vs Orcish Archer in S2~4, but its charge attack is risky. Only 1 recruit is needed due to loyal Haldiel and Simyr (Knight).
I would say there are 4 places where recruited Elvish Scout can't substitute Elvish Rider;Is this the only time the Rider is necessary? I employ Scouts but it's very rare they manage to advance because they rarely fight. Similar issue for Gryphon Master.S Elvish Rider is faster than Elvish Scout. It is the only unit, if quick, that can reach the Mother Gryphon on turn 4 in S10.
A Gryphon Master is the fastest flying unit. It is extremely good at clearing the fog/shroud to find enemy units in S19c, 20a, and 20b.
S4: Recalled 2 close-to-advancement Elvish Scouts advance to Elvish Riders, and they join the water battle to help Merman Fighters.
S8: Some distances between 2 villages are 11. Only quick Elvish Rider (11 MP) is fast enough to move from one to another in 1 turn.
S8 distance:

S15: Just in case all Dwarvish Fighters are non-quick, recall an Elvish Rider/Outrider to save the ally leader.
I agree that it's always a challenge to provide enough XP to relatively weak units such as Elvish Shaman, Elvish Scout, Gryphon Rider, Thief, etc. I try to have a clear strategic goal in mind when I use these units. It's a pain to use weak units without a future plan.
Elrian as Silver Mage
After capturing far villages, Silver Mage can come back to join the battle through the villages near the main army. Silver Mage's magical 9 × 4 attack is good at damaging enemy units without killing them to give XP to other units.I do wonder, specifically for Elrian, do you make him a Silver Mage for scouting/village flagging reasons? I end up rarely using Silver Mages for actual fighting so I'd think he'd be more valuable as a Mage of Light (or even a Great Mage) and simply recruiting a different Mage to become Silver.S Silver Mage can hop around through villages and earn gold. Also, it can help the enemy leader assassinations in S9 and 12.
D Arch Mage is just another Delfador. Silver Mage's speed is preferred for Elrian.
"Elrian (Silver Mage) + A recruited one (White Mage/Mage of Light)" seems better than "Elrian (White Mage/Mage of Light) + A recruited one (Silver Mage).
Reason 1: Loyal units are often recalled on turn 1 to save gold, but healers can wait until turn 2~3 because healers are needed only after taking damage.
Reason 2: It's not necessary to have 2 healers in some scenarios. 2nd loyal healer can become redundant.
Reason 3: A quick healer (6 MP) can move 3 cave hexes in S15~17 while a non-quick one (5 MP) can only move 2 hexes, and Elrian is never quick. Therefore, when a Mage is recruited in S3, it makes sense to pray that it is a quick one, which has a 50% chance.
These are my reasons, but I'm not denying other playstyles.
19abc 20ab paths
To be honest, I couldn't care less about any bonuses in 19abc & 20ab. They are all cool and fun, but are they crucial in beating the campaign? Absolutely not. So, it's okay to take any path. I originally chose 19c to 20b path because it seemed the fastest in speedrunning due to just escaping in S18. Later, I found out that it was the easiest and the most gold-friendly path anyway. The difficulty is probably 19b20a > 19a20a > 19c20b. In general, fighting weak but many units (19b) seems more difficult than fighting strong but few units (19a, 19c) because keeping the defensive line against many is challenging.So is the reason for taking 19c and 20b more gold-based than for the sake of the bonus? I typically choose the Swamp instead because of the Void Armor which I put on Li'sar.B Grand Marshal is the only level 4 unit with leadership ability. Even level 3 Konrad/Li'sar with Sceptre of Fire receives +25% damage boost.
Statistics: Posted by Orek — 26 minutes ago