You've mentioned several times that when he hurts, you hurt. Is the reverse true? When you hurt, like the current situation with T and S, does he hurt with you? Does he have empathy with and for you? I understand that he had apologized for past poor behavior but has he actually changed his behavior?
Has he done anything to address his anger issues? And promising to change does not count. Apologizing does not count either. Has he gone to counseling? I realize that you love this man and he is a good partner to you in many ways.
BUT, this is his problem to resolve. You can't fix this for him, you can be supportive of course but this is his problem. You are not at fault, you did nothing wrong. You are not the problem. T and S are not the problem, swinging or polyamory are not the problem. R's rage, passive aggressiveness and so far refusal to deal with his emotions in any constructive way are the problems. He is the only one who can resolve them.
And you show many of the classic signs of being in an abusive relationship. You minimize the impact of his behavior on you, you emphasize the infrequency of abusive actions and words, you have taken on all the emotional burden of anticipating, responding and soothing to his emotions, you put a higher premium on his feelings over your own. You emphasize the good things in the relationship while tolerating completely unacceptable behavior. You rush to his defense when others point out that his behavior is unacceptable. His behavior is abusive. Yes, he is more than that extreme reaction. I realize that we only know what you have written and we are all more complex than posts on a forum. But what you have written is very disturbing at best.