With respect (that’s not sarcasm, you are clearly an intelligent, rational, reasonable person) you may be overthinking this a tad.
I don’t think you’ve committed some grave faux pas which will cause other hunter trainers to shun you. I think what you did was fine. We’ll never know what Trainer B’s reservation was - maybe Trainer B screwed Trainer A over on a horse deal and Trainer B is righteously nervous about close association. I just think that in the future, you’d be better served by approaching Trainer A first. That’s all.
As for all your wonderful stories about your students, I have a lot of the same stories about my mentors and students in hunterland. My first mentor absolutely found me other jobs and horses to ride. When she felt I had outgrown her program, she encouraged to start my own barn and sent me lots of business to get me going. We traded students back and forth for years. I had another mentor who took me along with her and introduced me to her instructor, a v. big name, so I could start working with them and continue to grow.
On my own, I sent students and clients to other professionals that I thought were better fits frequently. In one case, a wonderful client with three horses on full board - they had outgrown my program, they wanted to show in the As, and show every weekend. I recommended another barn and facilitated the introduction and the transition and was very happy for their continued success. New barn/trainer always gave me credit for giving them a good foundation in my program.
The only one that was sticky was a kid who hadn’t outgrown my program, but thought she was a lot more advanced than she was. But we reached a point that because of her belief, I was no longer an effective instructor for her, so I found another situation for her.
My point is, hunterland is not La Cosa Nostra. Weird and insular, sure, but there’s nothing sinister. (Or shouldn’t be. If there is, get the hell out.) Trainer/student relationships are more formalized than in dressage or eventing. It’s just a courtesy to discuss your plans with whom you’re currently working with; but it’s not an edict and the transgression is not punishable by shunning.
Best of luck to you as you advocate for your pony jockey.