Also, they mostly use the arrival information for planning purposes so that they know how many people to expect and can staff the desk, bus queues and bus routes accordingly. As long as you have your DME ticket and arrive on (or after) the first day of your hotel reservation, you'll get on DME. So if the OP's friends literally make a last-minute change and cannot call DME before their plane departs, there's no reason to worry.
The only *major* exception would be if you're arriving in Orlando in the late evening. The DME desk is scheduled to close at a certain time (I think it's 10 or 11PM), but if they know they have passengers arriving on an 11:45PM flight, they'll stay open for them. But if they don't know you're on that 11:45 flight, you might get to Orlando and find that all of the DME staff have gone home. (The same thing holds true for anyone who finds their flights majorly delayed.)
As for luggage, all bags coming off of all airplanes at Orlando International are screened by the handlers for the yellow DME tags. Those bags are then routed to the DME luggage facility. Once they arrive there, DME only cares about where a bag is going, not where it came from.
The bags are then held in the DME facility until at least one DME passenger ticket from that reservation has been scanned as having boarded a bus. Then, the bags are released to be transported to the Disney hotel.
So long story short, your friends should have no trouble at all in using the DME system, as long as they're at least registered as guests on DME and have tickets issued in their names. And if their flights change, it's worth it to take a couple minutes and call DME and let them know. (There should be a number in with the info that comes with the tickets and tags).
Oh, and the more-important flight info that DME needs is your *departure* time when you're heading home. They need to know when to schedule your pickup time. But that can easily be changed while on your trip. (Though they suggest making sure they have your info at least 24 hours before your departure time)
-Rob