I think TLK clearly exists as a trope on OUAT. It’s just that the writers are not consistent because it is inconvenient for them to follow their own established laws of magic. We’ve seen in Skin Deep that the the dark one curse clearly is a curse that can be broken with TLK, but it stopped working when Rumple pulled away because he didn’t want it then. Later, however, Rumple’s dark one curse should have been easily broken by TLK with Belle once he decided that he didn’t want it anymore. Yes, it’s inconsistent, but the writers don’t care because of PLOT reasons. They just want to keep Rumple as the dark one for yet another season because they really don’t know what to do with him.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy