No your doors don’t just unlock if the site goes down. Do your locks open if you lose your key?
Yes, your system and any automation will not work if no internet connection. There are 100 threads already discussing this.
If you want to “Spoof” the ip, or redirect the traffic via proxy, feel free. Its just relaying messaging to the server. You still need to figure out what to do with it. If you figure out that, let me know.
As for the bandwidth, it depends on usage. Sending camera images up/down to ST will increase the usage, but the radio messages and checkins aren’t too bad. Just log into the IDE and watch the console log for a bit to get an idea of the local traffic up to the server.
Or sniff the packets from the hub, and see for yourself.