Iris

2. The opponent is most likely Laslo, any Orc (it's bad if it's Haghesh), or a Demon (bad because of the many Imps), possibly an Elf. Definitely not a Warlock, Mage, Necromancer, or Dwarf; my opponent doesn't like or know how to play them.

Perhaps this strategy is most suitable against Orcs. They have many melee units and no way to remove Vampirism. There's essentially only one ranged unit, not counting the Cyclops Goblin Throwers, but he's not very durable. The only way to control a Barbarian – the Frightful Howl ability – won't work on a target with Vampirism. However, Goblins with Magic Resistance and the Horde Frenzy ability are still dangerous if they get the chance.
Humans have the same Adepts who can dispel the spell.
With Elves, the main thing is to deal with the ranged units, but Unicorns won't blind Griffons.
Against Demons, there will be problems with mana, so you need as much as possible.

I also wanted to take Leadership, even though my main force will be Undead, but I have two strong stacks, and Empathy and Inspiration for the Inquisitors won't hurt, plus if there's no point in Readiness, then take Battle to the Last and go for the ultimate ability "Relentless Force".

In the late game, the Knight's absolute defense is almost useless. You might destroy even 5 units of level 7, but I don't remember the exact calculation formula, and there are more than a hundred of them. And it's completely useless against smaller units.

7. Regarding magic control against the opponent, I believe that Haven is the most protected Castle against Dark Magic in the game. Paladins, Inquisitors (80% chance of removal), and the hero himself – that's already three stacks that can remove control!

Generally, that's true, but if the opponent is a Demon, there will be problems. The hero can quickly run out of mana, and only the units will remain as a hope. And if it's also Alastor, then the units will also run out of mana, and in the end, only the Paladins will be able to remove the effects.

And another question. Doesn't the removal of control with Purification or Antimagic depend on the difference in spell power?

The chance of successfully removing a spell depends on the level of Light Magic and the difference in levels between your hero and the enemy hero. It seems that if you have Expert Light and there is no difference in level, it should always work. This has even less effect on Antimagic. With developed Light, it protects against all spells of the first four levels, and with Expert Light – even against the fifth level.