For convenience, I load a GC with a few hundred dollars at City Hall and use that instead of a credit card. If I don't use the full value at DL can I use the remainder at WDW?
You should be able to; I've bought Disney Gift Cards at DLR and used them at WDW and vice-a-versa. I would just check the back to make sure (the ones I have say Walt Disney World, Disneyland, Disney Cruise Line, Disney Store, and DisneyStore.com on the back).