You can, but TetherMe won't work, and the cycle repeats. It's either your iPhone is deactivated and TetherMe works (which you can't use since you're deactivated), and once you're activated, TetherMe no longer works.
That would depend on the developer. Chances are that it won't be updated til the A5 and A6 devices are jailbroken since those are the major "clientele" that's being grabbed. There's no point of updating it now just to find out that it doesn't work for the A6 and iPads.
The main concern now for tethering apps is the fact that iOS 6 incorporates a stock personal hotspot, which may potentially damage their clientele pool as well as their economy since many carriers will waive the tethering fee if your data pool is large enough, meaning that's $20 less you have to pay per month for a "free" service as opposed to MyWi which costs $20 flat. That's $20 less you have to fork over for a carrier approved tether access.