I had exactly the same thing happen to me. I'm now three months into a relationship with my mono boyfriend, Mr. A, but in the first month, there was a miscommunication. He thought he'd told me he was going on a date with someone he'd started seeing around the same time as me, but he hadn't. He said that night that the date confirmed his feeling that there really wasn't a connection with her, and they didn't even kiss. (Previously, they'd slept together.) However, my green-eyed monster EXPLODED.
Indigo, my fiancÚ, was hurt by the fact that I was jealous. I was encouraging him to spread his wings and ... fly. I was happy when he had dates, and wanted all the juicy details. I loved hearing about other potential interests.
So why jealousy over someone new and complete compersion for the established relationship? Certainly not because I don't care for Indigo. Quite the opposite. I was so secure in our relationship, that I truly did want him to find happiness elsewhere; I knew he wasn't leaving.
So yes, your A and B points were exactly what happened to me.
The bad news, I couldn't kill jealousy. The best I could do was communicate with Mr. A and trust him to let me know of any propects.
And then last week, something unexpected happened ... Given the nature of Mr. A's career, it is likely he will have to leave our city permanently within the next year or two. We were talking about this and while it elicited some of the usual sadness, I also started to feel compersion. His career is his primary love, and I know when he leaves it will be a better life for him. I had the beginnings of that "If you love something, set it free" feeling, which I've had for all of my true, healthy, mature loves.
Waiting it out with open communication is probably the best you can do. When your relationship grows deeper, you may just find yourself stumbling happily into compersion.