1) No. never saw anything extra there.
2) Yes. The damage mitigation is absolute, this is why I love the ability
3) This is the mechanic of Demon portal: Total Leadership = Leadership*(1+0.05*Int)*(1+0.01*(Summoning+Items)) . Spell lvl 1 summons lvl 2-3 creatures, lvl 2 summons 2-4 lvl creatures, lvl 3 summons lvl 3-5 creatures. It's random, so you can try placing the portal in a different place if the result is not satisfactory. On the other hand, Mystic Egg is not related to the intellect, simply to the leadership of the hero and the type and lvl of creatures that may spawn (use common logic - it's anything that can hatch - spiders, snakes, griffins, hayterrants, dragons).
4) When your army's leadership outweighs theirs (I'm uncertain by how much), they usually try to even the scales with armageddon. The workaround is to stall. Simply put your units in a place where they won't be able to retaliate much and have them defend till the enemy wastes his supply of mana.
4.5) Fire resistance only reduces the damage by the burning post-effect. Armageddon inflicts astral damage, and only now in the gift bag a few items that grant astral damage have been introduced. Otherwise, there is no resistance against that type of damage, so it will hit anything with full power.
- on a side note, if you cast armageddon, your units take only 50% of the damage. It's still a notable amount, though.
Hope that helps