No brilliant ideas, but I do have one thought. I often read P's words or actions as if she were me. She's not. It's plain as day to me that if I said or did XXX it would mean YYY. Even though I KNOW that P is different, it is still hard to really believe that if she says or does XXX, it might mean something different entirely.
Quote: my feeling that he keeps choosing himself over the R.
Ask, don't ASSume what his actions mean.
Can you be warm and pleasant with S even while this issue is unresolved? Maybe contact S and let him know that you want to discuss the issue further, but you need some time to sort out your thoughts first. And that you don't intend to give him the cold shoulder in the meantime.
