Can dismissed companions be trusted with my loot?
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.