Heroes 5.5 WarGame Edition - Tactical Edition.

I didn't quite get the idea here: how does a hero get new spells in the mod? After getting basic Light, will I get 1 random low-level spell in my book, or is there no Mages' Guild as such here, or what? And after getting advanced Light, will I get another 2 random ones? I'd like to discuss this in more detail.

No, the Mages' Guild works just as before. Using Light as an example:
Tier 1 - Evasion or Bless for 0 mana
Tier 2 - Skin or Might for 1 and 2 mana
Tier 3 - Haste or Exorcism for 3 and 5 mana
Tier 4 - Teleport or Antimagic for 6 and 10 mana
Tier 5 - Resurrection for 15 mana

Bonus Light spells are not in the guild (formally these are circle 6 spells):
- Regeneration for 4 mana - granted for the combination (Light + Summoning)
- Vengeance for 8 mana - granted for the combination (Light + Chaos)
- Holy Word for 12 mana - granted for the combination (Light + Darkness)

Combination means that both schools must be leveled up to at least the advanced level. This hasn't been implemented via scripts yet (because I am currently working only with duels), but it will be — and such a system is already built into duels for distributing spells between heroes.

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But because of one spell, you start to fear her like a DD. I mean, if she had some kind of spell at the beginning like Light's Punishment, and an attack animation like a monk's strike fell from the sky and dealt light damage, then I'd be quiet. But as it is, she buffs and buffs, and then at the end: top-tier DD.

Still, the key difference between divine vengeance and chaos is that chaos is applied before the clash begins, reducing the enemy's numbers, while vengeance is applied during or after the main fray.
That is, Chaos is like ranged fire, while vengeance is like melee damage. In this regard, Chaos is fundamentally more dangerous — it can win games under any conditions since it doesn't require preliminary clashes where you might lose more than you gain.
The whole question comes down to numbers: how strong should vengeance be. And here the fundamental point is that I believe vengeance no longer depends on spell power, meaning it performs consistently well but cannot be boosted above average. In the original, vengeance depended on spell power, and that's perceived quite differently.
Furthermore, Vengeance has become expensive (8 mana at the original rate is 32 mana), so you also need to save mana for it, and it's unlikely you can spam it without a significant boost to knowledge.

I agree with Darkness. Only now has it been opened to my eyes that light and darkness are not equivalent, because strengthening your own warriors and weakening the enemy's over time is not the same thing. Truly, truth is born in dispute =)

No worries, I also considered them analogs for a long time, just like attack and defense. The prevalence of mass spells played a role here — after all, mass weakness is indeed similar to mass skin in effect.
But single-target and area-of-effect spells change the emphasis. Balancing very strong buffs and debuffs in WGE showed their fundamental difference. Greatly boosting damage for one stack is not the same as greatly reducing damage for another stack, and not even the same as greatly reducing its survivability. The former is more dangerous per unit of time, as such a pro-buffed squad can deal a lot of damage to several enemy squads, while a debuff only weakens one.

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And where can I see the current spells and perks? They are on the first pages, but from the date I gather the formulas are no longer valid. Perks for skills are of particular interest to me. About 15 pages back, I read that you gave the orc the ability to summon manticores to help before battle (is this still current?). And what happened to the Barbarian placeholders? Can they be replaced? I would definitely suggest replacing them. And what about the dwarven runes? Top 3 things I hated in the original:
3rd place: resurrection rune
2nd place: berserk spell
1st place: a legion of free gays in a fully built dwarf castle

You can see them by scrolling back through the pages sequentially and skipping unrelated topics.
I'm not working on perks right now so that I can tidy them up and update the first page. First, I want to finish the duels.

But in short:
The Barbarian placeholders have been deleted entirely; instead, a new Barbarian skill will be added — Resistance (+10/20/30% unit health). This skill will have 3 perk branches — against light, darkness, and summoning. Each branch is a corruption (doubling the cost) and weakening (halving spell power). That is, now against any school of magic (except Chaos), you can put a maximum of 2 specialized perks, while the base itself (Health) protects well against Chaos.

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And what about the dwarven runes? Top 3 things I hated in the original:
3rd place: resurrection rune
2nd place: berserk spell
1st place: a legion of free gays in a fully built dwarf castle

Dwarven runes remain unchanged for now, as I haven't worked closely with racial ones yet (and they are disabled in duels).
Specifically regarding the resurrection rune — the resurrection percentage is adjustable, allowing you to freely strengthen or weaken the rune if desired. Resource prices are also adjustable.
The idea of limiting the number of runes that can be spent per battle by some limit of resources allocated to that battle seems logical, so players don't hoard resources for the "finale" and create imbalanced situations, while not trembling over every extra resource spent during a push, using runes more actively (for this, I'm thinking of linking it to dwarven racial mining — i.e., resource production depending on hero level).

In another project — MMH5.5, the issue was approached differently, and they simply removed the imbalanced Greatest Rune from the game, which allowed runes to be reapplied at triple cost. But I like this perk, and I believe it's better to create a proper context for it (resource deficit per battle) than to simply cut it. These aren't orc placeholders that do nothing but waste a bunch of levels on simple enemy magic weakening (and if you guess wrong, it's a dud).

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Regarding specific bonus spells:

4 Regeneration = Light + Summoning
8 Vengeance = Light + Chaos
12 Holy Word = Light + Darkness

4 Sorrow = Darkness + Chaos
8 Vampirism = Darkness + Light
12 Wave of Death = Darkness + Summoning

4 Earthquake = Summoning + Chaos
8 Hive = Summoning + Light
12 Raising = Summoning + Darkness

4 Fire Wall = Chaos + Summoning
8 Freezing = Chaos + Darkness
12 Armageddon = Chaos + Light

Actually, I wanted to tie Sorrow to Darkness + Summoning and Wave of Death to Darkness + Chaos.
But the issue here is that Darkness + Chaos are the native schools of demons and dark elves, while Wave of Death also hits dark elves, meaning this spell isn't very useful for the latter.
Now, if it were possible to make Word and Wave always hit only enemies (unlike Armageddon), then it would be a different story. But technically that didn't work out.

Shield is a guild spell because summoning spells are already taken (Earthquake, Hive, Raising).
Technically, it could have been granted for Light + Summoning instead of Hive, but Hive fits elves too well, while I would like Shield to be actively used by all races who have access to Summoning, and not just for the elven combination of Summoning + Light.