I don't pretend to be as experienced as others who have put up some excellent posts here, but I might offer one idea...

About the revoking of the safeword. If it is done because your wife wants to please you, then I can think of two possibilities: if you think she's really had enough, you can tell her the scene is over, period, but still make it clear that you love her and it's not her fault her body isn't up to the play levels you both fantasize about. This kind of reassurance is important for emotionally vulnerable or insecure people, and it might help her make safer choices about when to use her word. (And yes, that might leave you unsatisfied a couple of times - which sucks, but has the upside of demonstrating to both of you that you take the responsability for your sub's welfare seriously.)

A second possibility is one I found on the thread about safewords. Someone suggested a system of signalling by holding up different numbers of fingers to let the top know how close the sub is to safewording. Or simply having two safewords, like "red" and "yellow," to signal FULL STOP versus EASE UP THAT'S TOO MUCH. That would allow you to scale things back somewhat so that you wouldn't have to just cut a scene short.

Good luck with everything!