Was this a 2 story with the return between the levels are you positive those two grills were supply and return and not two returns. Can not use a wood chase for a supply, return only allowed. The stain on the carpet looks like a piece of furniture was resting there which could account for the discoloration appearance.
Whether a Ranch or Two Story, Basement or Crawl, what you are showing and/or describing indicates problems that I would recommend to my Client to have checked and remedied by a Prof. HVAC contractor.
Pic#1 S/A & R/A next to each other is an obvious problem which should be readily correctible if there is access to the ductwork in the basement.
Pic#2 Same thing as above, though a little more costly and time consuming to correct, but necessary.
Pic#3 I assume is a shot of a supply plenum or trunk line (hard to tell), mainly because of the round pipe out of the top with a damper handle/clip visable. If that is correct, wer’e not allowed to use duct(grey cloth)tape on supply ductwork, and haven’t for the past 20+/-yrs., in our area.
You can get away with more centralized return design, if done correctly. It is not always best design, but can work.