Sorry to be a little late to the party, but I am not clear on what exactly you want to do.
Do you want to separate the first floor from the basement and then use the basement for a crash pad for when you visit?
Or are you looking to live in the first floor and basement but then rent out some of the rooms?
If the basement is not a permitted space you are asking for trouble renting it out (or subleasing it with the first floor) I personally would perhaps think about moving your elderly parents into the first floor so that it is easier for them instead of going up stairs all the time. That way since there is a connection to the basement there is not issue with you crashing down there.
If you don't want to do something like that, I would cut the basement off from the first floor and just rent the first floor, leaving you the basement to crash in when you want.
Or see about getting the basement made into it's own separate unit now that ADU are allowed in Mass (although the law may not kick in until the new year)
Also your town may now require a certificate of occupancy to be issued before you can rent it out and they may have something to say about this basement connection anyway.
Just my two cents as they say.