I can't really address the first part about selling the house on paper and not transferring the NFT.

I figure this thing would have cellular access as well as Wi-Fi. So if your Wi-Fi was to go down, then the cell network would be used instead. And those generally use different ISPs for fiber and often get restored first or dont go down at all since they are commercial contracts. In the event of a total internet cut, it is well known that a house does not change ownership very often, so the lock could be programmed to not accept any new keys for a period like a day. The lock would accept only the old key during that time like a cooldown period