I think even without the devil's deliberate interference,their feelings towards each other will always end up dragging everyone into this hell again. Dietrich and her son would sacrifice themselves for FL but FL might do anything to save them which would definitely allow the devil to offer a deal to her and Johannes is deeply entangled to her to an extent so he'll be there to make her his again this time too, kinda creating a timeloop. If I remember right, I think there's an implication about timeloop or things like this happening again and again.
Basically endless suffering. Wow.
A part of me thinks it's better for her to just kill her son, so both of them will be free but at the same time, what if it's another trick from the devil? How are they even going to escape? With the power of love? Seems a little too late now.