Hello,
Adding return air is not normally an issue. Normally the size of the registers should be less than the return grilles. When it is the opposite the system does not reach maximum efficiency. The only thing I can see as an issue, is if one return travels a long distance and is needed to cool a room that is otherwise not part of the house. Like a garage room or servants quarters.

The point of connection should be infront of the airhandler. It doesnt matter where IMO.

1-This new return line will be installed above the subfloor & approximately 2' from the return air box into the living room.

2-If I understand corectly, this will not affect the existing airflow of return ducts located below the subfloor from existing rooms in the house.

3- The supply registers in the room will be less than the new return grill.

4- The point of connection will be on the right side of the return air box.

Does this sound OK?

Designing a duct system is almost magic. While I can say it won't affect the existing ductwork, without seeing the duct system, I cannot guarantee it won't affect a room or more. A lot of this has to do with how the system was designed originally.

My house originally had return air ducts in each room. It did not work until the return air was also added to underneath the air handler. From this, I would imagine the individual return air ducts are pretty much useless, but I have seen airflow into several return ducts. I doesnt make sense.

I think I may have a return air problem. If I add up the total square inches of supply air openings it's almost twict that of my return air. I know one of my returns has been floored over several years ago. The entire duct system was a total hack job thats been taped and screwed back together from previous owners. I haven't fixed much of it yet because so much of it is in a nasty crawlspace.

Scenario: Century wood Frame 2 storey home with modern gas furnace and air conditioning supplying both levels. Tho 2 CARs are located on the ground floor I suddenly realise there is no CAR on the upper level. Should one be installed.... what are the benefits...??? What is the principle of CARs...??

I take it CARs is Ceiling Air Return(s).
I would install one or more CAR in the top floor. There may be a problem running the ductwork.(?)

Do some research on the linked page(S).

udarrell _ 2007

Here is a link that might be useful: udarrell.com