Resort A can transfer your luggage to Resort B, leaving room f
In the car for the ECV. Just put whatever you'll need for that day/evening in a small bag you can throw in the trunk; your luggage may not get where you're going, some time after 4.
Respectfully, how does your mother manage everyday life other than the mobility impairment? Seventy isn't generally old. If she can navigate around town and function well, she likely could manage getting to/on a bus alone, and meet you at a park.
How many people in your party? Maybe one person could travel with her on the bus.
What about, um, having the lead person on the reservation drive to the new property with the luggage to check in, while everyone else takes the bus with your mother.