Etiquette-wise, inviting only one half of a couple, especially a couple that lives together, to a wedding is rude. For him to receive a single invite rather than an invite addressed to the both of them (without prior discussion of all parties) would be a social slap in the face.
Yes, that etiquette is based on couple-centric, even mono, standards, but this IS a wedding. Some couple-centric thought applies, even in a poly universe.
So I wouldn't consider his original actions pot-stirring. They didn't want to invite her, so by etiquette standards they should not invite him. If I were R I would absolutely want to know why the invitation was extended only to him and not to me, and him telling her, "I don't know why" would be disingenuous and a lie.
Unfortunately everyone couldn't put their big-boy and -girl pants on. In a perfect world, they all would have been able to talk about how M didn't particularly want R there, but in the interest of having the OP there they'd invite them both and expect R to do the gracious, mature thing and stay home. It does stand to reason that since R doesn't like M either, she wouldn't really mind not going to her wedding. It sounds to me like she just got possessive because her connection to the OP wasn't being recognized socially.
MC went to his cousin's wedding without me. And planned to go to his best friend's wedding without me, though my pregnancy issues got in the way of that plan. And conversely, I've sucked it up and gone and played socially acceptable when he wanted to go to a wedding of people I didn't particularly care for.
So either way, if he wants to go, the OP should be able to go. Either because R gets over spending one afternoon/evening without him, or because M recognizes that having R there will not drastically alter the happiness of her day. If he doesn't go, then I don't think his wants are being considered enough, only everyone else's.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack