Some people have done loops through free co ports from a spare extension if you had free ones, but it can cause issues with locking co ports.

Extension groups do have ucd option, so if you call one of the phones it jumps around to other phones in the same group. As for all ringing, no ring groups on that system

Yes you could ring a bell and and have a pickup code or button to answer it. Only issue is if two calls come in at the same time the second one will bounce back to the VM or give busy tone

“I have not failed. I've just found 10,000 ways that won't work.”