Coming back to the "overleveling" due to additional monsters granting XP... that's why one DOESN'T use combat-XP as D:OS does.
Pillars of Eternity tried to use monster variety as difficulty too, and it kinda works since it doesn't give XP per kill. On the other hand some people whined about it, then they added "bestiary XP" and since you fill those quicker on higher difficulties... well... good systems ruined by crappy gamers.
The same would be true here.
The BAD sollution would be to add scaling (hell no).
The GOOD sollution would be to simply use objective XP, not combat XP. Then you can alter an encounter per difficulty all you want, and don't have to worry about any of it at all, whatever the composition everyone gets the same reward.
