Does that even count as light for that purpose? Like, I've got it on her, and next to her people still got the curse and still got disadvantage from darkness.

Like, is it a bug, or is the light aura just the anti-undead effect and not actually light?

Technically in 5e, there's no reason they can't. Undead can benefit from potions just the same as anyone else. And given they have to make a potion to transform into a lich in the first place, I'd say they probably are skilled alchemists, so it'd not be out of line to say they make some.

Previous editions probably had other rules against it though, and it wouldn't work for pathfinder I believe, as most healing potions have a positive energy trait (which harms undead rather than heals).

I mean, think about how many farmers have turned into level 20 John Fighterman. She's got an army of level 20 souls who can all attack 8 times in the first couple rounds each.

Unless you're talking about the playtest, DnD you double all the damage dice, not just the base weapon dice. Sneak attack, smites, anything that's not locked away behind a separate saving throw (like poisons) or just a flat damage boost (like +5 to STR) all gets doubled.

PF2e is great though, would love if one of my games switched over some time.