You can "rent" a wheel chair (manual one) on board if you notify them in advance that you would need one. They hold a deposit until the end of the cruise. You could use that instead of trying to tough it out standing in lines. On CCL, they even let my mother in law take the wheel chair off the ship (maybe that was what the deposit is for??)
Most definetly, ask to be allowed to "skip" the lines. The crew are very understanding and will do their best to help you. You would not be the first passenger that needed the little extra help.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013