Can flex duct be used in place of a normal return plenum? I've read it can so long as it is not the only return in the house but not sure how good the source was. 8" is probably the biggest I can use but the plenum takes up 2 joist cavities, so run 2 flex ducts? Thank you.

I'll make a very long story short and jump to the bottom line. The HVAC contractor did such a poor job with the ductwork that the unfinished basement never gets above 65º all summer. And that's with it being on its own zone that's never been used. I could seal all the leaking joints and...