I have a 2 story house, 4 ton downstairs, 2.5 ton upstairs- 3900sqft- in texas. This is 1960s constructions so bad insulation, leaky, etc is a given. I've added more insulation, sealed can lights, sealed all around, better, but still not great.

The downstairs duct work was in bad shape originally- had 2 supply ducts COMPLETELY disconnected spewing a/c into the attic. Had a local A/c company come out and inspect everything (blower testing) and they did aeroseal on the downstairs supply ductwork (original mostly sheetmetal ductwork-most of the downstairs ductwork is inaccessible due to 2 story setup). 3 of 4 returns are in wall returns (between the studs, unlined). Per their post-aeroseal testing, the supply side went from around 300 to 70 (?cfm) leakage, so I'm happy with that. However the total system (supply and return) still shows around 20% leakage (sorry for the incorrect terminology, don't have the report in front of me)- and the report said I'm still losing ~1ton of cooling (of a 4 ton system). I asked them about sealing the returns and they didn't seem to say much other than not wanting to do it. Since then I've contacted aeroseal who said the returns could be sealed. Maybe I'm wasting money on this, but I'm trying to make this house comfortable and this still bothers me:
1)My concern is there are large holes in the returns sucking in 150 degree attic air- anyway to inspect the returns? Run a camera up them from the in wall returns?
2)Shouldn't they be able to aeroseal these also? My understanding is they could, it just takes a while. Especially if we make sure there are no massive holes, aeroseal should seal these well? If not, I'm not opposed to some sheet rock work if it means I get back 1 ton of cooling...
3)I paid good money for the aeroseal on the supply side, but feel like only 1/2 the problem was addressed- why is this so difficult to fix?