pekoms is not gonna betray moma..especially after the mugi just tried to kill her.
also he isn't even in the mirror right now as far as we can see.
personally i don't think luffy is gonna beat katakuri..at best i think he is gonna disable him for enough to grab brulee and flee away through a mirror, sealing him and his troops inside it for the time being.
Pekoms does not need to be in mirror to be pulled into it later on by Luffy using Brulee again.
I also have my doubts considering Luffy beating Katakuri and I see scenario you described as likely.. Lets wait and see. Luffy should get a win this time though because against Cracker, Oda made Luffys win very questionable and now we are heading into arc concentrated on direct confilict with a Yonko Kaido so.. Luffy escaping after questionable win would leave bad taste in my mouth and disbelief in any sense of Luffy heading to Wano.