Zombie Room

What happens if 15 zombies reach the circle?

You have to try again.

If you happen to get mob lag where they don't move, just windmill, anything else will teleport them to the circle after a while. There's 2 double spawns, so you can let 2 of them slide and focus on 1 of the doubles.

Try again from the beginning?

For that room, failing results in the switch lighting up again. So yes, you have to start the process of that room again.