Your topic isn't blunt and discussion heavy. It seems like two people working things out rationally.
This is the only point where I would disagree with what you propose - your arrangement does not have to be (or at least start out) "balanced and reciprocal." If you can find common ground, whatever it is, that satisfies you both, then that's where you go. It's one thing that I have found to be true in our journey of nonmonogamy and forming a poly family. At first hubby didn't play, only I did with my boyfried/ex-fiance because of my jealousy issues. Later, I realized how stupid I was being and how much I enjoyed knowing, then watching, then participating with other women having sex with my husband and boyfriend.
So go with what feels right and don't strive for an artificial goal like balance and reciprocity. And BTW, as I have gotten into my thirties, I can understand Mrs. E's need for cock more than ever.