Definitely go for the handicapped room. It will make it safe and easy for you to use the bathroom facilities.
As to leaving the wheelchair in the hallway--I'll probably get flamed here, but the hallways are not particularly wide. People leave strollers, etc. in the hall and really take up space. If this were in the US, it would probably be against the law (blocking fire evacuation routes, etc.) I find it to be rude. I understand the need for the chair, but that's one of the reasons for the larger handicapped cabin.
The ship is actually good from an accessibility standpoint (as is WDW). You need the extra space and features for your well being--have your doc fill out the medical form and get the handicapped cabin. You won't be taking it from someone who might be totally wheelchair dependent.
DCL will prioritize guests needs, and if there are people who must have the cabin (as opposed to you for whom it would be desirable but not essential), they would get it first. However, on most cruises, DCL either keeps the handicapped cabins open or sells them (late) to guests who don't need them as they have more cabins than they usually have guests who need them.
The servers/stewards try to clear trays from the halls as soon as possible. In fact, there is a request from room service that you call them to pick up your room service trays when you are finished so that they can keep the hallways clear.