The way I see it there are only three people that have ever had possession of the curse, Malificent, Regina, and Rumple. The fail-safe had to have come from one of them.
It is really unlikely that it came from Malificent, since Regina said that her punishment was different from everyone else’s. It seems if Malificent would have put anything in the curse that might have benefited her that Regina caught it.
Rumple, I can only see him wanting a fail safe for two reasons, because Bae has gone to another realm or has died. But Regina didn’t know why Rumple wanted her to create the curse. He told her to figure it out. So I doubt that there was a Baelfire clause in there at all.
Rumple asked Regina for the “please clause” and she agreed to it…so that proves to me that she was capable of altering the curse.So I think that the failsafe is Regina’s doing in case it was her last chance to (literally) destroy the happiness of Mary Margaret and David, although I think that she would prefer watching them suffer if at all possible. Going into this curse she didn’t think that there would be any magic besides what she had brought, and she knew about Jefferson’s hat. I wholeheartedly agree with those who said that the failsafe is Regina leaves. Eventually even an Evil Queen could get tired of revenge.