I honestly wouldn't count on being allowed to take an in-park rental out of the park. To my knowledge, that is generally not allowed so it was an exception if someone did that. There should be courtesy wheelchairs available to borrow to help getting to/from the parking lot or buses, but be aware these are not always in the best condition and often are not where you need it (for example, they all might be in the parking lot when you are at the park gate ready to leave). Earlier this fall the WDW resorts discontinued offering courtesy wheelchairs for guests who needed one in an emergency such as an injury occuring during vacation. It is always best to either bring or rent a wheelchair if you know you will need one getting around from park-to-park and/or resort-to-park.
Enjoy your vacation!