>>1>>3 I'm a lot like you, my mom and I are very close, and I tell her a lot of what goes on in my life, since I know that she can give advice or encouragement when and where it's needed. However, I think that the key thing in a relationship such as this is discretion. Sure, tell her the situation your friends are in, tell her what's going on, but NEVER give a name. I learned that the hard way and now so have you. Ultimately, parents are there to protect us, and most wouldn't think twice about protecting another child from a potentially toxic relationship such as this.
In regards to A, of course she's gonna be mad, what'd you expect? She'd be happy that she had to break up with her boyfriend because of you? There is no logic in that reasoning. I'll be brutally honest with you, it wasn't your business, and it wasn't your information to share so openly, you are a large part of their break up, and you must accept the fact that she may never trust you the same way again. Her reaction is more than justified, given the circumstances. Did you do the right thing for her? The way I see it, and the way most others will see it: yes, yes you did. You saved her from a relationship that could easily have turned toxic. But the way she sees it is something completely different: you've forced her to break up with someone she cared for and had dreams to be with.