AC question

Yes, having a return improves the air flow. It's basicly blowing and sucking from the area as opposed to just blowing.

It's a good idea to check out all the ducting in any house you buy to make sure it's all going someplace. It's not unusual to find something like this, or worse.

Since the return duct goes somewhere, is it possible to connect it into the return system? If so and it can be done easily, I would do it, as it can only help.

Assuming the door is open I doubt it would make much difference in the summer. Sucking air from the room would draw air mostly from the hallway and not the incoming duct. Main problem is incoming duct is too small or blocked.

If having returns isn't important for cooling, why bother having them upstairs at all?

you might be absolutely correct. the hallway door is always open, and the duct feeding the air is somewhat smaller than most in the house.

To help mix the warmer air in the winter between upper and lower stories.