My advice - suck it up and know you are doing the right thing by not going to the wedding. Despite your feelings for M, R is your spouse and it isn't right for you to brush off R's dislike of M that way, as though it shouldn't matter. It should - but as you said, that is a subject for another post.
As far as what to buy M & D for their wedding, I strongly recommend you discuss this with R. Don't just guess what she would be comfortable with - talk to her about it.
That's the short sweet answer to your specific questions, but you really should address what the issues are between R & M, and address them with each of them separately - openly, honestly, and without letting your "hit the jackpot" excited feelings cloud your judgment.