If I’m understanding you correctly (and please feel free to correct me if I’m not), you’ve been able to connect your account and get everything up and running with another device since this freeze? Your only problem lies within the other device, the one still harboring the 10.0 iOS update?
If that is the case, then more than likely the problem is with the device itself. Not as in it’s bad or incompatible or anything like that, it just means that there’s probably something going on with it that will need to be figured out. Sounds like something just needs to be flushed. I just don’t know how to go about doing that with iOS when, from the sounds of it, Apple took the ease out of the users hands.
I’m sure support will be able to get you in ASAP, though. At least they’ll do the best they can. I could be wrong and there could be something more going on, but I hope it’s a simple fix. Good luck!