Per a previous post (Cellulose insulation odor), we’ve noticed a smell of newspaper after installing cellulose insulation in the attic.  The suggestion was to check the returns since the air handler is in the attic.  I went back up and re-taped the two sections of duct that connected the return to the air handler.  The smell still remains.  So that leaves the two ceiling return grilles in the hallway (1970’s ranch).  There is also a small off-shoot that is “ducted” via the gap between two ceiling joists (i.e. there’s a piece of sheet metal over the top and on the end) that goes to a couple of open ducts in the basement.  I pulled the two filters on the hallway, and have attached some pictures.  I hadn’t noticed / remembered the ducts in the basement (we moved in a year ago) until I was trying to find where the sheet metal run between the joists went. 

1. The fact there is no filter on the ends of the basement ducts is probably why there is dust in the return, correct?  Should I just cut / tape a normal filter over the ends of them?    
2. What type of insulation is the black stuff on the interior of the returns?
3. Can / should I try to remove it so I can seal between the return and the ceiling?  I believe I’d taped all of the visible seams before installing the cellulose, but don’t think I was able to get below the return (i.e. the short vertical section between the ceiling and return). 
4. Or should I try to go back up and spray foam / mastic the return-to-ceiling connections?  Is there a best practice for sheet metal to wood joist?
5. Any idea what the bright white thing is?  It’s between the two ceiling returns in the hallway. 
6. Does having the new R-8 duct wrap on the outside of the return, plus the black / original insulation on the interior of the return create problems?  I wasn’t able to look inside the hard duct on the supply side (there’s a long section of sheet metal and then flex ducts run to each of the ceiling registers), but it was wrapped with R-8 also.

Thanks for the thoughts / suggestions!