I though the Trial of the Chicago 7 was class. And Baron Cohen was brilliant in my opinion. The accent is Massachusetts yes. And he nailed it. A sad situation that the current world is no different to the one they experienced then. In particular the Black Panthers.
If you're Bostonian, then you'll have the upper hand on me to say whether Cohen's accent is 'authentic'.
For me, I've heard the base layer of that 'accent' too many times and enough to hear when he has difficulty in forming his vocals around pronunciations. He's a million miles off a the Boston native Wahlberg family or even the soft lilt of Boston influenced Dana White.
I say that cos I listen, have heard accents and have done my best to place them for as long as I can remember.
As for the rest of your post, it would be pretty interesting to see if there's a genuine attempt to make a 'Panthers' movie, rather than one that skirts the edges about that period of time.
*Actually writing that gave me pause for thought I had a look to see if there actually was anything other than docus on the BP Party.
There's a film due to be released next year called
"Judas and the Black Messiah" with our own Daniel Kaluuya playing 'Fred Hampton'
Having just sat through this trailer, I've got to say DK is magic, the trailer gives me goosebumps and DK looks like award winning probability!
Place your bets, people!