We have been members of DVC at OKW for over 10 years.
Welcome Home to you!
Member Services handles all special needs requests for DVC members (or anyone staying at DVC using points). If you actually need a Handicapped Accessible room, we have been told those are guaranteed (because, as you noted, if you need that level of accessibility, you will not be able to stay without it). If, by accident they have overbooked HA rooms, they will find one for you at another resort (that is what we have been told and it is not likely to happen).
Other medical requests, like first floor or non-smoking for medical reasons, are not guaranteed. But, those requests are filled before the general requests for the same thing. Whatever request is most important to you should be listed first (because they assume that is why you listed it first). They will try to fill at least that request. The more requests someone has, the more difficult it is to find a room that fits all those requests. In 10 years of staying at DVC resorts, we have always gotten at least our first request.