Working for a major airline, I will tell you seat selection is the biggest complaint. Picking a seat ahead of time is like requesting a gound floor room. It is a request, not a guarantee. If at any time they change the specific aircraft to a different number aircraft it is possible, everyone who has requested a seat will have it changed. Having said that, any decent airline will always accomodate any disabled person or child. It is a matter of safety to have these people seated with their traveling companion. If you check in at home, you already have your seat assigned and only in a rare instance will the seat be changed. Also, you are right, those who do not have "held" seats will be the first to be bumped in the case of a flight being oversold if there are no volunteers to get off the plane. Which brings up another point. VOLUNTEER!!! you will get a voucher for a future flight (translate: free airfare to WDW) and a seat on another flight. Sometimes only a couple of hours later. Ask the agent for the alternate flight. Also request 1st class. Never hurts and you might get it. Or ask for a 1st class certificate on a future flight. Not all airlines or all cities have them, but again it never hurts to ask.