Maybe not, but I haven't seen confirmations one way or another.

I don't know about wireless standards to say whether or not it's possible to selectively disable regions or devices.

For the phone, the Gov would just have to tell the carrier to turn off the acount.

You can literally buy phones from vending machines in the UK.

Yes, but Assange cannot step out of the embassy to buy one himself, and he's fairly paranoid. There are probably only very few people in the world from whom he would accept a phone to enter all of his precious passwords, if he is willing to use a phone for that at all.

It's probably all the same to him. Assange is likely clever enough to assume all his traffic is combed through by smart people, no matter which data transport he uses.