I'm thinkin about giving Aranea a good sword and shield for when she runs out of mana, but I'm not sure if she even can.

Followers apparently can, enemies never seem to though. Krosis must have shot a few hundred fireballs in succession before I finally brought him down.

I'm quite sure they do. I've had enemy mages come at me with the dagger after taking quite some lightning damage.

Yes they do. I've played dodge with some enemy mages before and after a few spells they had to charge me with whatever crappy little dagger they had, then of course they died quite quickly.

It seems like she never uses her shield; she'll pull out the glass sword I gave her and always use some spell in the other hand

well my follower runs out of arrows, so i assume some can run out of mana too, I know npcs certainly do, seen a few of them charge at me with daggers. the thing you may need to worry about though, is would she be any good with a sword and shield? or would she just charge in and get killed. If she's not wearing armour, maybe a bow is a better choice.

I think some enemy mages have basically bottomless pools of mana, maybe there is a limit but after blowing a few thousand mana attacking me and still slinging the spells it doesn't seem to be a small one. Which would be fine if I could loot there awesome near infinite mana gear for myself, but it always ends up being a crappy robe. On the other hand svcker enemy mages run out fast.

I think some do and some don't. Bosses don't.

NPC's choose the best weapon in inventory so she's probably better at spell-casting and one-handed weapons then she is at blocking and two-handed weapons.

i have a staff that drains an enemies magicka but if they have no magicka it svcks their health, and when i use it on enemies mages or warriors, it takes a few seconds then their health starts draining rapidly, meaning they have ran out of magicka.