Hi we were on the QM2 in November to NY and although neither of us are disabled there were quite alot of people that we saw in wheelchairs and the ship is very well designed to cater for people in wheelchairs and even have special lifts for wheels chair users. There are lifts to all floors but there are a couple of quite steep slopes around the area of the planetarium but there is always someone round to help you so I am sure that you will not have any problems. It is a very big ship and even with 6 days and nights at sea we did not get to see it all. I would recomend the spa costs about $50 dollars for a 4 day pass but well worth it!!!
Hope you have a great cruise.
I think you will find your needs are more than catered for on QM2. Most of the docks in the Carribean islands are quite flat and very pleasant making them easy to walk around. Contact your agent or Cunard direct to check exactly what they are able to offer in terms of excurions. We are doing this cruise also. Kind regard.
I find Cunard very good for wheelchair users just the lifts a bit of a bind but you find that everywhere have you informed Cunard of your on chair i found them very helpful no waiting on boarding or diseimbarking at southampton.