not that bad.

for project, PM has the absolute authority. so do whatever he wants. For other stuff, your boss, M has the absolute authority. Your job is to make those two aligned to what you want to do.

at this point, the conflict is between M and PM. You are the middle person. have those two sit in the same room and discuss this fully. I can assure you, they will not yell at each other and they will come up with a solution by themselves. all you need to do is to execute. document everything of course. If something went wrong in teh future, you are not the one responsible for it.

Thsi whole issue started at incoporating B. You could have convinced your boss that B can wait for the next round. A is good enough for his contribution to the project this year (and this is the underlying issue, he wants to be involved in teh product). If I were you, I would use the opportunity to insert myself into B, to make it better so that you would not have headaches in the future. With PM, you should have suggested adding B feature into the product as another round of improvement and let PM know that you are helping make B adaptable to the product. Everybody would have been happy.