Yucata vet here, but never participated in YPS. I've added myself to the Master spreadsheet and put a 'p' under column heading '22'. Is that sufficient at this point?
How can I tell which events I can still register for?
Events 1-18 have already begun, so in principle you can no longer register to them. Events 19-42 have not begun yet, so you're free to register in any of those.
One exception is event 17 (2p Vikings). It has already begun but not a long time ago and one player abandoned the tournament at the very beginning. So here there is an open place. If you're interested in this particular event just tell me so.