Hey there. I'm so sorry to hear about the heartache you're going through.
The absolute number one issue here, no doubt about it, is R's anger management issues. I thought that at the end of post 1, before the swinging even started, so I see this as having nothing to do with non-monogamy. Even if you closed your relationship back up tomorrow this would still be something you'd HAVE to deal with to be healthy together.
Throwing things at his partner and shoving, especially when he's (presumably) the physically stronger one, is menacing, scary behavior, and it's emotionally abusive even if he's not leaving bruises. How would you react if you saw T treating S that way?
I'm not saying he's doing it on purpose, but he's an adult and he has a responsibility to get control of himself, for your sake, for his own sake. Has he ever been to therapy? Would he consider it? I'm not a fan of ultimatums, and you clearly don't want to leave him so I'm not going to tell you to walk out the door and not look back... but you may well have to threaten to leave, and maybe even follow through for a time, in order to challenge him to wake up and get healthy.
Healthy adults don't hurt their spouses, they're not incapable of maintaining a stable opinion about something important in their life, they don't break boundaries and then turn around and belittle their partner when she hasn't done the same, they don't use other people as emotional leverage to threaten and bully their partner (which is what I see him doing by continually declaring that he might just cut things off with T&S).
This is not a healthy man. There's a reason you'd be mortified to let T&S see this side of your relationship -- because you know it's wrong wrong wrong. Victims of abuse are at great risk for ending up in abusive relationships as adults and this is an abusive relationship, despite all the times he's loving and sweet. It's up to you to decide for yourself... are you willing to fight to break the pattern? Are you willing to believe you deserve a life where you're not treated in ways that would make your friend recoil in shock and sadness?
I haven't talked about swinging/poly yet because It seems beside the point, but since advice on that is what you came here for, I feel I should at least touch on it.
There is no reason for it to be inevitable for one member of a quad to eventually end it. Why should it be, if all are happy? Great article about a live-in Utah quad: http://www.cityweekly.net/utah/artic...current_page=1
Right now I don't want to encourage you to pursue poly, because your base isn't nearly stable enough for it and you'd just end up with broken hearts all around when R exploded and couldn't handle it.
But, for the record, it's not so uncommon for people to move from swinging to poly. It takes a lot of communication and respect and trust and patience and more communication to make it work. If you want to learn more about the fundamentals of how to have successful relationships in this sort of situation, I highly recommend the essays at www.morethantwo.com
Please seek help, Awake. Best wishes.