From what I recall, the process of the body becoming undead consumes the soul, correct? At least *usually*?

Hello,

Correct - that's why the rift between Beltinians and Nerothians exists; for an undead to rise, its soul acts as a catalyst or fuel to power the transformation.

If memory serves, this doesn't apply to Zombies or Skeletons (ie: Graveblight spells) which use magic and summoned spirits to animate bodies.

Yes, that's correct as far as I know. To remain as an intelligent undead costs you your soul. To have your body later used as some kind of animated or mindless necro puppet is a seperate thing.