You have nothing to do with the PA/PB waitlist for upgraders. It is simply a fare class waitlist (similar in theory to the upgrade waitlist) that will clear you into F as soon as an A seat opens up. I believe it is FIFO.

I would be surprised if no A opened up by day of departure, unless F sells out, but they will almost always open up A before any award or upgrade seats. You should be safe, don't worry