I'm aware that all nodes are already registered, but what happens if a current node goes offline? Does a spot open up for registration?
Once a node is registered, I do not believe it becomes unregistered even if it stops running. No additional slot opens up until the next registration period.
The actual node registration gets filled rather quickly (usually 5-20 minutes), so you have to be ready to go at the bell. Even then, its been roughly a ~30% chance to get a node registered due to the volume of people trying to register at the same time.