Magical Express stops at all three All Stars as needed. Even though some guests staying at 'Sports and 'Movies take the (non-DME) 'Music bus back from the parks and walk through the parking lots if that bus comes first.
No way to know for sure. The routes are very fluid. There will be at least one prior to Music going to the resort...and possibly two. Going back to the airport there will be at least one stop and up to two or three after. Just no way of knowing for sure. Figure on it taking up to 90 mins to get to Music and you'll be fine.