First of all, Rumple isn’t gone for good, and Belle has never given up on him. She’s not about to move on from him so easily, especially with the son of the man she loves. Neither has Neal given up hope of a second chance with Emma. Neal would not “go there” with Belle, the woman he knows his father loves, even if he were really grief-striken. Neal isn’t the type to try to come between two people who love each other; he’d do the right thing just because it’s the right thing to do. Also, I don’t think Belle would ever look at Neal in the same way she does his father. As far as crossing boundaries, this is a show for families, not a show just for adults. The film that introduced “Mrs. Robinson” is for adults for a reason.
"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