The teleportation spell is random. In general, it teleports the target into the stomach, but it can also do anything. It is not possible to predict this. ( random can be modified in custom.ini )
Centaurs aren't finished yet, they don't know how to fight and haven't learned how to swallow prey.
The MC can't swallow alive prey other than with an arrow or the teleportation spell (he can also use enemies' weapons and their swallowing abilities). But he'll learn new skills later. ( maybe with quests )