Academy of War -- Enter for Training ?!?!

[Arralen]

.. are the XP requirements per level, or cumulative? (i.e., does it take 50 or 65 XP for a fresh unit to get 2 stars?)

Per-Level requirement.

..do spells that hit count for XP?

AFAIK, no.
Neither do missile hits.

.. priests ..

Can easily get into the Hall of Fame when fighting Ermor, but not because of spell hits but because of "killed" (banished) longdead etc. - they get 100 kills or more quite fast

.. some of my mages getting bonuses to their magic ability ..

Haven't see this.
AFAIK the only way to get +1 death is by coming back via "Twiceborn" spell. The mage might end up with "feebleminded" or "mute" affliction, and therefore useless, as well, though ...