Does attacking enemies with enchanted weapons or spells that damage stamina/magicka actually cause them to stop casting spells and do power attacks? Or is it like with the arrows where they can shoot infinite arrows but only have a few on them when you check their corpse. I've heard you can even give followers one arrow and they keep shooting it indefinitely.

For spells yes. Once mana pool is gone, they will go melee.

It's also true that followers will shoot indefinitely even if you just give them one arrow.