Just got back from our first trip with my wife and 2.5 kids (0.5 to 1 due next week). Wow, M5 is a quiet, comfortable, and zippy hauler! OK, it is automatic; with our habit to change cars every few years, we had to think about selling it...
A few more answers:
Windows in the sliding doors do roll down, and they roll all the way down!
I am not sure about older MPV; I think it is much smaller than current one. In Europe there is a nice competitor to M5--Opel Zafira:

Maybe GM will re-badge and bring it in, who knows...
Echo is for sale for 7950.
Thanks for your comments!