NOW it would be a lie. It is too late to "unknow" that M said things about not being comfortable with R.
The more tactful thing to have done if she really did not want to invite R was to give OP a face saving reason -- like having struggles with balancing the list against grooms family and budget. Who DOESN'T have those struggles in a formal wedding? Or call up R herself and present R with a face saving reason so there is no social sting -- "I have a problem balancing my list... would you mind terribly helping me? I'm sorry I cannot manage two even though we want to share our joy with you. Would you and OP prefer just one invite for OP or no invite and just get together as a quad to celebrate at some other point? It would really help me out and I'd be most grateful!"
Then it is making it THEIR generosity in helping her. Not her cutting either from list.
Then they can feel good, and she can feel good knowing she's got more time (under less pressure) to better sort our her rocky her relationship with R without compromising her wedding day.
If M complains to him again about feeling uncomfortable about R -- gently but firmly tell her -- "This is my wife. I am sorry you feel that way. Please don't put me in the middle. Sort it out directly with her." More assertive, and maintain healthier TMI boundaries.
Rather than going home to R to say "Guess what? M said .... about you."
Same thing in the other direction. Tell wife gently but firmly -- "This is my girlfriend. I am sorry you feel that way. Please don't put me in the middle. Sort it out directly with her." More assertive, and maintain healthier TMI boundaries.
When you add these two together:
There are no secrets between the four of us -- everyone knows everything.
Long story short: R finally met M around June of this year, each having plenty of preconceived notions of the other. They don't get along.
that's not helping the struggling tier to find a happy medium to get along together better. How did they get preconceived notions of the other if they never met til June? From OP and D? Who are each coupled to one and dating the other?
I'm not saying OP and D ought to be lying to M & R. Just be more firm with TMI boundaries. If M & R ask weird questions say so. "Um... that's a weird question that puts me in an awkward spot. I'd prefer you sort that direct with _____."
Juicemonger's wants are not being considered because he isn't being assertive about articulating them.
But wedding invite or not -- His wife is still his wife. His GF is still his GF. Take heart. This will blow over. Weddings are stressy. Hang in there.