Gas and Oil Home Heating Furnaces - Return duct fix

I've been slowly fixing the duct work in my quad-level house. The install was done poorly (plus it about 14 years old). I've have 2 question about the return air ducts.

1) This house has a return ducts in about every room and in their in the ceiling except for the living room its in the bottom of the wall. Problem is that didnt do good job sealing it, they didnt use ducts, they let the wall\attic act as the duct which mustly drawing air\dirt from attic. I was wondering install of trying to install duct in the wall. If I could just put return in the ceiling above where the old return was and run duct over to the return trunk and just seal the wall up. Thoughts?

2) Part of the basement is finished, and has 2 feed ducts no returns, air right now has to go under the door up the stairs and under another door into the den before it reaches a return vent.
I was wonder if it would better to take 1 of the feeds and hook it to return in the attic. Thoughts?

thanks

On question #1, utilizing stud space for returns is not only illegal in most areas, but poor installation practice for a couple reasons. You can effectively put in a ceiling return however the problem may be the duct size to give you the correct amount of air volumn and velocity. Too much volumn might and you will override the velocity of the other ceil returns. Put in either a butterfly duct damper or an opposed-blade return grill to regulate air volumn. On question #2, you've got to decide whether or not taking away one supply will make a big difference in the basement. If not, go for it. Air allways takes the course of least resistence and undercut doors are the norm (2 1/2 to 3 inches).