It is not for certain whether she will return in FTL or SB or both. Bae returned only in FTL. August as of yet is seen only in SB. There is a possibility of a FTL appearence of Belle.
Didn’t Robert Carlyle say in an interview that we would probably see “Gold and Belle” rather than “Rumpel and Belle”?
Belle is alive in Storybrooke whereas Baelfire as far as we know isn’t.
I have the strangest feeling that Baelfire has been in contact with Belle. No evidence, just a feeling.