As to why Rumpel is powerless in NL, I think it has to do with Nl running on belief. Pan has magic that negates other magic, because it is based on belief not just emotion. That is how the cuff/taser things worked. It amplified the disbelief of Greg and Tamara (or their belief in the superiority of science) to the point it cut off Regina’s powers.
Phee already pointed out that Rumple has his powers but I love this idea about the taser.
So Pan’s threat to Rumpel is, if Rumpel keeps trying to get Henry, he will “believe” away the Dark One curse. Rumpel then has to be strong enough to choose to give up the power, to be undone. It’s a threat because of course, for Pan to summon enough disbelief to kill some magical thing, especially one capable of inspiring irrational fear, is not easy, that’s why he has all the kids.
My mind. It is blown.