Hi Freelancer - I know someone who is wheelchair-bound and only cruises with Princess. She has become a loyal customer due to the way they treat her. They bend over backwards trying to make her comfortable.
I have personally seen MANY crew members stop what they are doing and talk to elderly people that are in wheelchairs when they are alone. I'll never forget on one cruise in particular, I was sitting in the pool area writing post cards and observed an older lady in a wheelchair who was all alone. Not one, not two, but 4 crew members stopped by to chat with her. They didn't want to sell her a cocktail in a souvenir glass and they didn't ask her to move her chair out of their way. They simply wanted to talk with her, ask her if they could do anything for her and laugh with her. You'd think these crew people were her children the way they took care of her!
The Grand Class ships are very easy to get around in, IMO, but I probably haven't noticed the details of getting around like you would. The layout is very simple. Our two sons had the ship down in a matter of an hour or two. I think the only confusing part is getting to the aft dining room, in which case, you have to use the aft elevators only.
I'm sorry to say I haven't sailed with Celebrity or HAL - two cruise lines I've missed mostly because of our sons (ages 11 & 13.5) and their preferences to ships with video game arcades. But, they are in my future as soon as the boys are off at college. Maybe someone else here could help compare the cruise lines for you? Post a message! This board needs more posts!
Sorry I couldn't be of more help!