Sounds like agents just didn't do a good job of explaining the situations. I'm not that familiar with the technical details, but I think it goes something like this:
(1) The two carriers are still operating separately.
(2) You made a ticket change with AA. Someone at AA didn't re-issue your ticket properly. Error in ticket. AA ticket, AA must fix it.
(3) US is the operating carrier. US IRROPS = US responsibility to accommodate you.
(4) You chose to take an alternative flight to BOS rather than PVD. You must eat the costs of travelling from BOS to PVD.
Not sure about the business availability issue. I'm somewhat surprised that someone who's UA plat and probably a somewhat frequent traveller is not familiar with these problems on mixed carrier itineraries or itineraries operated by an airline other than the ticketing airline.