Oh and at least in my sitch, I think we may need to see a C to figure out what kind of arrangement is workable and what we can really handle emotionally in terms of flexibility. The arrangement with the kids is IMO not the appropriate arena for proving oneself or exhibiting changes. You've got to be very matter of fact and consider what kind of communication you two can handle (flexibility requires more communication and you two struggle with that already).
These sitches often start out more rigid and then get more flexible over time (or unfortunately sometimes is is the opposite or it ebbs and flows). To me, in your sitch, the clearer the better; up front, negotiated and maybe do 3 months at a time in advance.
I like the idea of having someone else involved so you can check in and see how things are going and diffuse any emotional baggage.
