Most likely interjection of a Pope Nicholas would be in the 330s in place of Pope Mark in, IIRC, 336. That would give a short reign for Nicholas before his passing in 343, but then you could have the same popes thereafter.
Oddly, after looking at it, I think just increasing Nicholas' prestige a litte before 336 would make it plausible for him to be chosen. And yet, it is entirely possible that he would STILL become associated with Christmas.
Santa Claus could have been Pope and STILL the Santa we know today...though maybe he'd wear all white instead of red....