Your professor is being bullied, and he's telling you to go along with it because he doesn't have the guts to defend his student.
Treat it as a bullying situation. Why should you, your work & your career be victimised because your professor is being bullied?
And further, why do they all think you can be the ultimate pawn? You've to stand up and fight back but in a way that you can win. No point rocking the boat and losing. Do it quietly and hit hard in unseen places that hurt but they can't cry out. They have to smile and grin in public and back away from you.
My advise to you is to become a b*stard that they did not see coming:
- Gather some facts, know the weaknesses and pasts of the other professor and Mr A.
- Do not talk directly with the other professor. Be very nice to him if you meet him.
- Go and talk to A, face to face. Demonstrate that you have his weak spot in your hand, and you are going to squeeze if he doesn't back out. Give a deadline. Send him messages at odd hours that make him lose sleep. Keep the pressure up. When you have someone's weak spot in your control, they will find it extremely hard to think rationally.
- He might react according to stages of trauma/grief: Denial and Isolation, Anger, Bargaining, Depression, Acceptance. Be cold, know what he's going through and it's according to your plan. Be merciless, be relentless, force him to get to acceptance. Do not accept bargains.
He's got to be sure you're serious.
- Once your deadline is up, squeeze hard but not all at once. Act more than your threat. Don't show your ultimate cards though, promise further pain. Give another deadline, shorter this time. Keep the pressure on.
Be absolutely cold and logical. This is your 4 years of work. They want to play you out and profit from it? Well find their pressure points, weak spot. How can you hurt them without hurting yourself? Let Mr. A run to his professor and cry. If the other professor wants to meet you etc, avoid as much as possible, say all the right non-commital things and make him feel at ease, but behind his back, squeeze Mr A with righteous vengence.
Second, don't trust or turn to the system to help you. The professors are the system and they will burn you.
Third, don't expect the story to end here. Stories like this will be whispered behind your back and impact your reputation and career further on. Make a stand, win quietly or spend your career walking with your tail between your back, wondering "what if..".
Source: I've been successful when I was bullied at workplace by a boss, by doing something similar.
Good luck, I hope you win and report back. Cheers!