Be aware that the wheelchairs from the resorts may not be in the best shape. We used a chair from OKW last December, since we flew and didn't drive down, and didn't want to hassle with our own chair on the flight while changing planes.
It rolled OK, but it was rusty and worn. I've never been told by a CM or on the contract that a resort chair can't be used in the parks. As others have said, there is limited availability. It doesn't hurt to ask at check-in of one is available, and if not you can call an offsite company to have one delivered the next day.
As far as the busses, a collapsed chair can be loaded, but be sure you have a secure grip upon it, wheelchairs are heavier than most strollers, and I wouldn't want to be whacked by a flying chair if the bus makes a sharp turn.