I don't think folks get a HC room just because they request it. I think you need a physicians letter for
DCL to give you one of the rooms. However, if the room is not filled, then DCL will release it to anyone.
We had a companion chair on two previous cruises, yes you can get it in the door (ours was a small size) but it was not easy, usually took someone to hold the door and someone to push. It took a lot of finagling for just one pusher to do it. Just about impossible to push it past the end of the bed in the room shapes that have the bed first (Cat 6 and 8). Not a problem in Cat 12 where the bed head is on the inside wall, as opposed to the side wall. Even collapsed there is not a good place to store it, other than one of the bath halves (tub? if no drippy bathing suits) or in the closet if you do not have any luggage on the floor, or on one side of the bed in the Cat 12 type. We once had a room (6526) that was the first outside next to the Cat 6 verandahs. There is a jog in the hall that has the cabin stewards stuff in it. Our guy let us keep the WC, collapsed, there, when we were in the room. Kind of a pain for the cabin CM, but better than a cabin with three or four persons AND the chair, which is a little crammed, no matter what the category. Two people and the chair, yes, but more than that you will be struggling.
Also pick a location closer to forward, or best, aft. The center elevators are always crammed full. Aft has a clearer shot to the restaurants. Deck 9 is a pain to negotiate from forward to Topsiders. Should you be forward, Deck 4 is a good choice to cross the ship. And, check with dining when you board to make sure you have tables forward and not buried in the back. Anyone stuck in a chair will not see the "entertainment" unless they can stand. The entry for the WC for Lumieres (Tritons) is off the hall, not the staired entry off the lobby.
Carla