OP here. Since there were new posts, here is a summary of our billing issue noted above. Much more complicated, and more steps IRL, but some interesting outcomes.
PCP continued to stress that I pay the full amount (more than negotiated insurance amount), and that I solve the issue with Medicare directly. I called Medicare each time PCP pushed for payment.
Each Medicare call required a repeated explanation with a new representative, who had slightly different advice, but each time they confirmed that my responsibility for the office visit was $0, and finally sent a MSN (EOB) with that noted. I never received one earlier because they actually paid the PCP (twice) and rescinded payment when PCP failed to supply some requested information. One representative said the MBD could now bill me only 20% of the original Medicare negotiated amount.
Trying to better understand my financial responsibilities, I called the state Medicare ombudsmen office. They were extremely helpful during discussions, and advised I pay the 20%, mark the check “paid in full” and send via registered mail. So I did.
Interestingly, Medicare reps shared ( several times) that the PCP was issued multiple “assignment violations” on this matter. I asked for a written confirmation of that as well, but never received one.
PCP cashed my check, crossed out my notation, wrote both DH and I that they would no longer be our PCP, and sent the balance to collections. I mailed collections a summary of events, and phone discussions with Medicare with a copy of the MSN. Never heard from them again. No credit ding.
I was advised to submit a complaint to the state attorney general, or the state insurance department. However, without anything in writing to support my phone conversations with Medicare or OSHIP, I decided best to move on. Sadly, I noted later on online reviews of this MD office, that several others also had issues with their billing practices.
Was it worth the aggravation and multiple calls? Definitely not worth my time for one office visit, but DH & I always felt this MD billing office previously stretched the limits of what was acceptable billing and frankly, we had enough. I can be very stubborn 
Multiple “small” mistakes with multiple patients often add up to a pattern!