"Business Class would also include priority boarding"
Depends on station you are traveling out of. Example = St Louis has never given Business class ticket holders Priority Boarding - although they easily could. You get Metropolitan lounge in Chicago but no First Class lounge access in St Louis on same route - Lincoln Service = Amtrak inconsistency. Not even same station experience in a major terminal on same route.
"but I don't think Acela Business class gets access"
Correct only First Class Acela gets lounge access. You pay close to or above a Sleeper price for your Acela Business class ticket - but have to stand in a long line waiting to board. I was lucky in DC and New York to be front of line. Hadn't gotten SELECT+ yet and wasn't traveling Acela same day as my Roomette reservation = so no lounge.
"Do you get free non-adult beverages in Acela BC? Newspaper? Pillow and blanket?"
BIG FAT NOPE!! You pay a huge ticket price for Acela BC and have to purchase anything and everything you want. Acela Cafe car attendants are some of the most unfriendly I have encountered in Amtrak system.
"Also there is no reason for the service to be consistent nationwide."
And you are great example of why things will never change for the better!!