Thank you for your interest in this question. So what do you want? Feelings of jealousy, self-consciousness, sadness, and anger are prevalent in such an emotionally-charged situation. If these two had been honest and had wanted to not betray you they would have A told you they were going to date each other. If you want to stay friends, take a temporary but hard break.
First of all, please narrow this down to 1 question, so the extra questions bit has to go.
Ask Molly Ringwald: my best friend is dating my ex – I can’t forgive them
When your friend violated the ideal that you listed he destroyed your trust in him. Think about the reasons you broke up with your ex in the first place. I'm still not over him. Remember the old saying about not saying anything if you can't say something nice? Take a break from seeing your friend.