I cannot say for sure, but considering that even dragons respawn eventually (at the respective dragon waypoint areas) I would guess that mammoths respawn. Bandit camps, witch covens and even chests respawn (their loot). Skyrim is supposed to be infinite (repeatable quests, respawning loot/ores/herbs, infinite supply of basically anything.)
I see no reason why the mammoths shouldn't respawn, so I'm guessing they'll do that eventually.
I'm a mage. If I firebolt "them" I get gangbanged like a pornstar
You need to be a mage and have the impact perk. Duel firebolt between the two mammoths (our focus one so he dies quicker), then sprint by holding down Alt so that you continue keeping your distance. When mana is out, wait for it to charge again and just keep running. You should have no trouble staying away since Mammoths, while strong, are slow as hell, especially with the help of Impact stunning.
The larger your mana supply/mana regen, the quicker this works. If you're a very low level, this is still doable, but it will take far longer. The only way this strategy may not work is if you're a mage who is decked out in all heavy armor so that you have a very low run speed.