So I've beein running around questing with Uthgerd the Unbroken (lady from The Bannered Mane in Whiterun, I beat her in a brawl and now she's my pack mule). She has loot from three dragons and one dungeon, so it's pretty precious. Now I just started a new quest with The Companions, that gave me a new pack mule and I can't boss around Uthgerd anymore until I get rid of my current mule. In the meantime, I'm wondering if my stuff is safe with Uthgerd, or can her inventory be reset, like with unowned containers?
Uthgerd will keep your loot safe. Followers maintain their inventories forever, even when you dismiss them.
However, there's a bit of a critical capacity bug that will cause followers to start losing capacity, but it won't affect things they're already carrying for you (just new stuff). So while Uthegerd will keep your stuff safe while you help the Companions, you'll find when you recruit her again she won't carry as much stuff anymore.