Is this the location in which AVIS rented vehicles are to be returned inside JFK, if you are flying after leaving the car?

Yep it is

Yes the train which circles the terminals will get you there

No it's on the outside but you can use the Airbus to get to the terminals

This is the one you can walk to.

You can walk to the terminal from the rental station. So, I guess so? It's really close