I dunno that the failsafe diamond would have been Regina’s idea. I imagine it more like someone got in her ear and suggested that because Rumple couldn’t be trusted, maybe a backup plan was in order. I could see that person being Tink, working for PP. Purpose of the failsafe diamond was to wipe out everyone else and isolate Henry. Isolating Henry is what PP wanted. So if all else failed, this was PP’s backup plan to get the kid, not really Regina’s at all, even though she thought it was hers, (just like she thought the Curse itself was for her to get her revenge, but really, it was for Rumple to find his son…but really, it was for PP to get Henry, (if the Curse never happened, Emma never would have met Neal, and Henry wouldn’t even exist)).
Wouldn’t surprise me if the failsafe diamond is seen again. Regina picked it up in the mines, so it’s probably still in her pocket.