I think the question has not been authoritatively answered. The weight of opinion seems to be that the answer is 'yes, the papal act of canonization is infallible.'
How about saint Christopher? didn't they just recently say that he never really existed?
Saints were originally determined by popular acclamation. Since their existence is known only through oral tradition, no one can know for sure what aspects of their stories are true and which are legendary. Additionally, there are too many saints to give each of them a day, so St. Christopher got bumped for another saint.
Yes, but St. Christopher was never declared a Saint by the Pope. The traditions of various ancient areas are also preserved; St. Christopher was one such traditional Saint. IIRC, he was removed from the canon becuase there was suspicion he never existed.