Make an advance reservation for Magical Express. Print-screen it if you can. Technically that has been one of the airport authority rules that permitted DME to exist. If you happen to be one of the zero point six nine percent of the guests whose tax returns, er, Magical Express appearances, get audited and you can't even show an envelope on which you scrawled the name and city of the telephone reservationist you talked to, then you might be denied the bus ride.
Before calling DME, pick a flight, any flight, any airline, arriving in Orlando roughly at the time you will be down at the DME welcoming area on resort check in day, ready to go to Disney. The reservationist may or may not ask for that.
While you may well win an argument (here down at the welcoming area regarding advance DME reservations), you would save precious minutes of vacation time avoiding the argument.