I found this note on page 11 of the installation manual under upflow units: NOTE: Where the maximum airflow is 1800 CFM or more, both sides or the bottom must be used for return air. This is saying the bottom return by itself will be enough. There was note under the upflow unit section, but I assume it also applies to a horizontal installation. You want the filter and grill cover to be at least the size of the opening to the furnace. Making the grille bigger is not going to help if return itself is smaller. I don't know how this is going to work with the filter size you quoted. I would think you would need two 14X24 filters. It would be nice if you get get a 4 inch filter instead of the 1 inch. Otherwise you will be changing the filter every month. The 6x6 stud obstruction should be removed and you want a tightly sealed return duct. The return duct should not have a right angle. It should be a curved elbow to make the 90 degree transition. Sharp angles create more air resistance. How big was the original house and AC, and much area are you adding? Most houses don't have the properly sized duct work to handle a 5 ton AC. Do you have confidence in this HVAC contractor?

Covering vents is a bad idea with regard to air flow and noise and overheating your furnace.

I had two return vents with the standard stamped cover shown in girl wonder's post just above with a time stamp of Oct 18 at 9:59. They were noisy with a whooshing sound. When I had some HVAC equipment replacement work done a couple of years ago, I asked the contractor if something could be done to quiet them. He chuckled and said "Those are always noisy. I'll bring some others to replace them, you'll like them much better". What he brought is similar to the image shown at bottom right in the array in the first post of the thread, labeled there as "classic grid". My HVAC guy called it "egg crate". Whether that or any of the other open one, I think you'll find noise is significantly lower.

16” x 24” return couch is 4 inches off the floor open on all sides with an angled back I think it’s OK but not sure