You don't mention where the furnace and duct is located but being in a cold basement or especially an attic where cold air naturally falls, you could have some migration. In addition you may have a fresh air make up duct that is letting even colder outdoor air into the return duct from the outside. This you should check, some are wide open to the outdoor intake duct and can have a manual damper while some have a motorized damper that should be controlled with blower operation. Make sure the return duct doesn't have any leaks and check connections and cabinet for leaks and cracks for air to infiltrate. Most furnaces have lousy gaskets on the blower doors and do not seal well. You might try taping the seams with a good aluminum duct tape. The problem with covering the return is having the furnace turn on without removing the cover could result in unit control and/or blower damage, including overheating and fire.

Yes this house is pretty strange. Its an old farmhouse and when they put in the oil furnace (which we don't use) they just sort of slapped the registers here and there. We basically just stopped the cold air with batting and sealed it up with duck tape (neon pink). its already made quite a difference.

It would appear that somehow your fireplace insert is tied in with you regular heating system. You will have to go do a little crawling around and tracing of the duct work and block it off where it needs to be. You have to be careful though to make sure it doesn't affect the rest of your system. If you're not comfortable doing it or not sure you're probably better off hiring someone who will understand what they're looking at in your house. It's pretty difficult to see into your basement from here.

Are there other returns beside this one? A 6" x 20" return by itself would be too small for the smallest furnace or AC. If there are additional returns then partially blocking this one may be alright. Will the back of the love seat be placed against the wall? If it is then see if you can pin the back skirt so it stays up and keep the love seat 6 inches away from the wall in order to allow air flow.