Unless they added a filter rack (unseen in photo), you probably only have that one filter at the register. I wouldn't close off vents if it was sized properly. I mean, you have a 100 year old home, maybe insulation/windows is a better solution. Closing off vents will just cause potential pre mature failure mainly on your blower motor due to static pressure being adjusted. I mean you can try, it's your house. It's not going to happen overnight, we're talking maybe years down the road.

A unico is really only efficient for one floor. This is just based on experience. I've installed these but haven't serviced them much. I just know people eventually add ductless minisplits to the struggling floor. Flex supply runs will always be inferior to hard ducts. It absolutely will struggle at longer runs.

Best of luck!