I'm in the process of replacing my furnace, some contractors want to make the return a square connection where it hooks up to the air cleaner, while others want to use a radius turn into the air cleaner.

Is there any real advantage of the radius connection over the square? Common sense tells me the radius would have better airflow but the installers prefer the easier square connection.

I also currently have no returns coming from the upstairs. There are two bedrooms in a finished attic space. Some contractors say I only need one return in the hallway (this makes no sense to me unless there were no doors) others say I should have one in each bedroom.

Are my assumptions correct?

Your intuition is correct. Leave the attic doors open and you'll be fine.

The benefit of room returns as opposed to the central return really comes from Cooling more than heating its essential for proper cooling less necessary for heating its up to you on how much work you want to do ! Good luck