Which creatures from the alternative upgrade options do you choose?

It seems a bit one-sided. You would take Zealots against Ossir or Red Riders against Necromancers? Well, Blue Griffons are clearly better in most cases, as they deal damage and don't just fly around aimlessly for half the battle. Angels are quite debatable and depend on many factors.

Zealots with Ossir is a separate case, and as you know, every rule has exceptions; it all depends on the circumstances. It's clear that Red Riders are not a good match against Ossir.
But I don't understand why Red Riders are bad against Necromancers?
If the Necromancer doesn't position his units correctly (or if the Knight is just lucky with the positioning), he can hit 2 key stacks (you shouldn't rely on the opponent being a noob, but still...). I think this is a worthwhile replacement for the Paladin's ability.

Again, a lot depends on the opponent and the Mage's upgrade. The Warlock will laugh hysterically at the sight of Elemental Gargoyles. Golems with infinite retaliation can decide the game.

To be honest, I just didn't think about the Warlock case.
But I prefer Magnetic Golems only because of their ability to resurrect through magic. If they end up in the thick of the battle, and the hero plays as Chaos, you can not only destroy enemies but also resurrect the Golems.

Info on Chaos with its own spells? How is that even possible? And how can upgrades depend on this?

If a Demon finds a couple of artifacts that boost magic power (once, I played with a friend, and he managed to dig up one of Sar-Iss's relics, which increased magic power by 6; the battle was intense). And upgrades depend very simply:
Better Imps, because mana goes to the hero, not to the Cave Lords (although if the hero has enough mana, you can take Lesser Imps).
Greater Imps are better for attackers (especially in a jump and with a blessing, with their spread), but they are simply not suitable for a Chaos Demon, because one jump and they will take full damage from the retaliation.
Firehounds are better for breaking through boxes, again for attackers (although they will also work for Chaos, the difference is not that big).
Succubi are generally excellent; you should take them without hesitation if the enemy is not a magic user or a Necromancer.
Cave Lords, although they cannot deal much damage to low-level creatures, are very good against large creatures and provide very good fire support to the Chaos hero (and summoners really hate them; Phoenixes cry :) ).
The upgrade for Devils is clearly better for Chaos, because having another stack of Cave Lords is a good thing, and for attackers, the alternative upgrade is better, because you can pull any creature out of any box and kill it with Demons.

Nimble without a charge is very weak; you should never take them.

I agree, they are weak, but doesn't the fact that they hit everyone on the way compensate for this disadvantage?)
Although... if the enemies surround him and don't let him move, the Riders will have big problems; however, the regular unit will also have many problems in this case.

3. Stealing ammunition from the Spirit is clearly more useful. A strong Human can decide the whole game. Stealing mana is needed less often and does not affect the game as much, unless it's against a Demon.

Yes, mana theft happens less often, but I take Ghosts only because of their higher initiative than Spirits.

5. I didn't understand about the Liches. For breaking through with resurrection, for the final stage, and with weakness.

Greater Liches can reduce losses at the beginning; their resurrection power compared to the damage of enemies (especially neutrals) is very high, allowing you to clear out a lot without losses and survive an encounter with the enemy without too much pain. But later, when there are already many units, the ratio changes in the opposite direction, and their resurrection becomes useless (unless you can resurrect Ghosts/Spirits to cover other stacks and remove retaliation; for other purposes, they are not very useful due to the small number of resurrected units). With Plague, Archliches are the opposite - at first, the plague is not so useful, then it becomes more or less useful. If we consider the usefulness of Greater/Archliches in relation to debuffs, then Archliches simply have a larger arsenal, which means they will be more useful (for example, with the Greater Demon - with its spread of damage, weakening is somehow more effective than weakness).