And in general, I was surprised that someone thinks that unicorns are weak and need to be buffed – they have initiative, magical defense, decent HP for their level, and also a blinding ability.

Magical defense is useless, initiative and blinding are good, but the point is not in the unicorns, but in the fact that, in general, Light Elves are not a top-tier faction, and there is no point in weakening them. And the problem is mainly not in the units, actually.
I saw unicorns as a fairly simple solution, as they are clearly not as favored at their tier as archers at their tier, druids at their tier, and ents at their tier, for example. Although, perhaps, Nargoth's idea is better. But with nerfing archers and dancers, a significant buff for fairies is needed. Perhaps, increase the magic of dryads (give them a higher skill level) and a special ability for nymphs? I think it would be good in its own way.

Regarding fairies, yes, there is an imbalance; I missed it during a quick review (in RTA, the difference is corrected not through stats, but through enhanced spells). You can follow a similar path, increasing the spellcasting and dispel magic skills by 1. The advantage is that stats are not touched at all (as if there are no changes).

Well, that's basically it, yes. That's why I say that symbiosis can also be strengthened – dispel magic and the swarm of wasps with the skill already look relatively decent (the swarm of wasps should push back on the initiative scale, if I'm not mistaken). But you need to "survive" until the nymphs' special ability takes effect – ents don't appear immediately, and there are also moments when their survivability is not enough.