Fortress versus Necropolis.

Summoning is certainly useful during the breakthrough. But it's possible to do without it, because by the final late-game battle, it will be nothing more than a waste of levels.

First, let's consider the absence of summoning. How do you intend to clear serious neutrals, like titans, without summoning and spiritual link?

Summoning, along with machines, is for beginners and rush games. If we take into account that you already casually assume the use of spells like summon phoenix or wall of swords, it's logical to assume that I will be just as likely to pick up berserk, mind control, or vampirism. Then, breaking through 8-12 titans without losses in weeks 2-3 (which is what I usually do, though only with "raise dead" without skilled summoning) becomes a mere formality.
If you rely solely on the power of "raise dead" through summoning, then that is certainly an advantage. But you can bypass this point, i.e.: level up Orson's BB or BDP — you can sacrifice general stone skin (it's easier for him). Generally, by the middle of week 3, about 400 skeletons are gathered on average, so together with single ghosts, alt-zombies under the ability, the hero (averaging level 15-20), and weak raise dead, this is quite achievable without any convoluted tricks. We cover our archers with a stack of 1 bone warrior — this is a remedy against all ranged units. That was an exception for titans :D
Other strong neutrals (including the utopia in week 3) are taken down as follows: take zombies (there are about 250 of them by the middle of week 3), 400 of the same archers, and single ghosts. Skeletons shoot, zombies stand and hit if necessary, ghosts provide cover. AI neutrals are designed to always hit ghosts and almost never hit zombies if they can hit ghosts. Moreover, zombies have very high defense stats under Orson — in week 3, 4 black dragons from the utopia took out 6-7 zombies who were standing under the "defend" command.

Next, here we lose phoenix, heavenly shield, and wall of swords.

But in exchange, we gain vampirism, mind control, and berserk. The only useful summoning spell in the finale is heavenly shield, but even that is depleted in 2-3 hits given such army volumes — compare that with the effect of vampirism and mind control. I won't even mention the phoenix — in the 4th-5th month, it's a pitiful gimmick that achieves nothing — and the enemy hero can take "deathly cold" from chaos, hit with a ring of frost, and nothing will remain of the phoenix, and others will be hit too... This isn't the 3rd week of the game. Any number of phantoms is extinguished by armageddon. And thanks to mass darkness, the hero, among other things, also acts more frequently.

Next. Mind control and vampirism are mentioned. It's not a fact that they will be in the guild, or there will be either mind control or vampirism if the second castle doesn't have the guild of darkness magic. What to do?

I probably misunderstood. Are we talking about the lands of outcasts, or a map where there are NO utopias AT ALL (in which all necessary spells are always found within 4 months), second castles, and libraries? But then what kind of late-game map is that? Heavenly shield also doesn't drop 100% in the castle, nor does wall of swords — some damn earthquake will drop instead, and that's it.

Third. Anti-magic is usually used against mind control.

Yes, such a practice exists. But will they really be able to instantly dispel both bears and jarls before they move under mind control? And the hero still has vampirism. He might not even cast mind control (I only use this against orcs in extreme cases), but will cast vampirism on another unit — then what? Anti-magic, by the way, is from light. Light, like darkness, implies mass spells. Firstly, there will be no time for anti-magic (since there is still divine vengeance), secondly, all positive light buffs will also be removed from the creature. Or are you suggesting 2 magics — light and chaos? But then how will they find time to hit with armageddon?

Fourth. No one also considered that the dwarf can also take chaos magic and cast armageddon a bit.

So, let's do some practical calculations:
The main striking-surviving (i.e., those who usually remain until the end of the battle and hit significantly) force of the undead: 1800 skeletons under Orson = 12600 hp, 1000 zombies = 21000 hp, 75 harbingers = 7650 hp, 50 dragons = 8100 hp. The dwarf hits with armageddon for an average of 900 hp (with spellcasting 29 — it's exactly that on average). Suppose he has pyrokinesis. Fine, 1800 hp. How many times does the dwarf need to fire to cause significant damage? Correct, at least 2 times, even under pyrokinesis. And let's not forget that the undead hero doesn't just stand still, and in that time will apply all 4 debuffs (3 debuffs and mind control) to the dwarves, and the undead troops will already be fighting in close quarters with the weakened dwarven ones, of which there will definitely be 1/3 fewer.
And there are tools such as reflection, ice shield (although I almost never equip it even against a dwarf), and undead magic protection instead of distraction.