... she'll be able to get a 9 yo boy into a shop when there is a ride in the offing! A game arcade, maybe.
I'd use FASTPASS religiously. I'm convinced that the reason Disney created it was to get more people to shop while they wait their turn on the rides. I think that it will be a good bargaining card for you in this situation. Get the fastpass first, then MIL can shop during the wait time if she likes. However, the trick to using FP is to stagger them; when the ride time is coming up, head to a different ride to get another FP, then turn around and go ride the ride you had the first FP for. The reason for this is that they limit how many FP's you can have at one time, but you can get another as soon as the time on the first comes up. The FP boarding time is a 30 minute span (or at least it was last fall), so there is plenty of time to do this. It means more walking back and forth across the parks, but I don't think your average 9 yo will care about that. It's no crime to split up occasionally, as long as you have an airtight meet-up plan, with a contingency backup rule.
Absolutely do the big rides as early in the am as you can! Use early entry if you can get your DS up in time, then leave that park and go to a different one (or to a lunch PS) by about 11:30. If you really like doing rides and MIL doesn't, maybe you and DS could do an e-night if one is available, while she takes it easy back at the hotel. Anyway, good luck, and lots of pixie dust; I have a feeling you will need it!