When DD was 8 1/2 months old she took her 1st trip to WDW. We had a really great light weight stroller that also turned into a back pack. (by Instep, 50$ at target.) She was the perfect age for it and it really made the trip much eaiser. Sorry, it will not hold the car seat. It folds down very small for busses, and we were able to change it from stoller to back pack with 2 people with her still in it. (being very careful!) We found that it really helped with some places that do not allow strollers, but you have to walk (One man's dream, the jungle treck in AK and play sets.) It also was wonderful for pardes, when she could be up high. (also good when the parks were very crowded and pushing a stoller can be difficulty. She had no problem with falling asleep in it and it also had an canopy. It sounds like you are set with the stroller you have, but wanted to let you know how well it worked for us. My DD also would not use a sling or carrier, but loved the back pack. She was ok with a on the hip carrier by OneStepAhead.
Also it depends on how heavy your DD is if you want to carry the car seat with you. BTW we did take one to a movie when she was young and it sits fine on the seats. If the baby is alseep I am sure it is easier to take the whole carrier with you then take a sleeping baby out of it and wake her up.