Since each school or district establishes it's own "spring break," it is a little hard to answer your questions. Easter week is generally "prime season" with high rates. In general, all of the March/April sailings will fill up. However, even with a full ship, it doesn't feel crowded.
There is no room service on the last day. You can eat in your assigned restaurant at the assigned time or you can go to the buffet on deck 9 at any time you'd like. The ship is well equipped for mobility problems. You will need to have your doctor fill out the medical form (very brief) if you are requesting a handicapped access cabin.
At
Castaway Cay, there are wheelchairs made for beach use. As to the other excursions, it depends on which excursion. Some have a lot of walking. There is a symbol by the description indicating how physical the excursion is.
DCL is happy to answer specific questions.
Getting off on the last day is easy. You place your luggage outside your room the night before by the specified time, keeping something to wear in the morning. In the am, you get up and go to breakfast--you are assigned to the restaurant where you ate the night before and to a specific time based on your dinner seating. Alternatively, you can go to deck 9 any time you'd like. When you leave the cabin for breakfast, you are out for good--take any luggage you have left with you. After breakfast, you go out the big doors to the luggage/customs area. Your luggage is arranged in groups by cabin number. You raise your hand and a porter comes over. He/she loads your luggage on a cart and takes you thru customs. You then proceed to your car/limo/DCL bus or whatever your next form of transportation is.
We've done many cruises on the Magic. Generally, the disembarkation is totally smooth. DCL asks everyone to be off the ship by 9 am. That said, occasionally Customs doesn't clear the ship and getting off can be delayed.....