1. In the final chapter, Race isn't in the booth, remember?
2. It doesn't actually explain the appearances and disappearances, per se. If you were hinting that PJ's ghost caused them, you might want to literally state that. Actually, I think we should have them escalate in frequency, length, and weirdness until the end of the whole MiST, and tie it into the conclusion, where all the "Poofy People" from the entire fic converge on Zahid, giving everyone a window of opportunity. Of course, MMK and Gavok will join in the Zahid beating, and as Zahid finally gets away, the doors shut. MMK and 'Vok reason that at least, this time they have the company of the ghosts, just as they simultaneously disappear for good, ending the fic.
How's that?