Skelemancer?

[Epi]

...reading monstats.txt from 1.10s results in some weird numbers (like fallen1 has 50hp and 50-90 damage regardless of difficulty). There must be some kind of multiplier, but what is it?

Yes, there is a multiplier indeed, stored in molvl.txt (indexed by monster's level):

MinHP(normal) = monstat.minHP{monster} * monlvl.L-HP[level] / 100
MaxHP(normal) = monstat.maxHP{monster} * monlvl.L-HP[level] / 100

For NM/Hell, use L-HP(N) or L-HP(H) columns correspondently. For Realm non-Ladder games use columns without 'L-' prefix.

I have a question; if I hire a Might merc at level 1 (heavily rushed) will he be better than a might merc hired at say, lvl 23? Better stats/higher aura lvls or so?

No. Only difficulty where you hire matter, not his level.

-Epi.