In order to open up the return pathway we added a wall grate/return below the blocked area.
When we got here both return ducts tied into this “box I’m the ceiling - one on one end and one on the other. (And as I mentioned, the larger of the two return cavities was mostly blocked.)
In the 1st photo below you can see that “box” plenum where the two returns met, and there was one small grill (with no filter) collecting the air. For the time being I just enlarged the opening in the box/ceiling, then added the wall opening (2nd photo below):
New opening in wall to let larger unit breathe:
These, of course, are just bandaids...