Regarding which boy was Rumpy's undoing, I agree it appears to have been Pan. However, according to onceuponatime.wikia, this is the prophecy from the seer:
You will be reunited with your son and it will come in a most unexpected way. A boy . . . a young boy will lead you to your son, but beware Rumplestiltskin, that boy is more than he appears. He will need you. Do what you seek, but there will be a price, the boy will be your undoing.
The boy who led Rumpy to his son was Henry. IMO, the prophecy is only talking about one boy, and there is no wiggle room for it switching to be about another. I think both Pan and Henry played a part in Rumpy's undoing. Henry did because Rumpy couldn't destroy his beloved son's son, and Pan did because, well, he was just beyond evil and incredibly selfish, especially towards his own son.
Now, on a slightly different note, I've heard people say that Rumpy must still be alive because we saw him in the previews. The only time I saw him in the previews, it looked like the scene when he and Belle embraced after the gang returned from Neverland. I saw nothing else that looked new with him in it. Did I miss something?