Also, what happens if the site goes down or the internet drops? Are the doors unlocked and the system just dies?
any option for localized backup brain? Spook the ip and crack the hash to direct all outbound traffic to a local box masquerading as a smartthings server?
The communication uses very little bandwidth, the actual amount will vary by the number of apps, schedules etc you have installed and the number of devices you have.
As for the rest of your question, there are numerous posts about this already. If the internet is down you cannot control your devices from your phone, smartapps and scheuldes won’t work.
I want to Spook the IP too! 
I think ST would be scared of that.
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.
yes I know it’s spoof. autocorrect.
I would monitor network traffic if i had a system. These are pre buy questions.
And no patrick, my doors don’t unlock when I lose my key. But being that i don’t have a system to test with, I don’t know what the failover state is when connection is lost to the hub/cloud.