[dad911]

I'll throw out my 2 cents.

Almost 20 years old, that unit reached end of life. I'd change it. Without AC it's a simple swap, easier than a 2.2 in a 1971T. Probably quicker than the time you will spend debugging.

I'm also not a fan of common returns. I don't know the layout of the house, but imagine adding a few additional returns in other rooms would reduce the load and help balance the system better.

Flexible duct is cheap. I'd replace that return before cleaning. Probably less than $100.