I agree to an extent with Sawn. Since you love the girl and looked forward to them getting married, I would just be happy they did get married, and move on. I think you are holding on too hard to the expectations you had of their big day and the hoopla associated with a bigger wedding, based on the conversations you had with the girl some time ago. You feel let down because their plans changed, and cannot let go of that excitement you had built up. Personally, I think it is awesome the two grandmothers were their witnesses, and that sounds like something I would have chosen to do, if I had been allowed to follow my choices. However, my wedding was railroaded by my mother, her best friend, and my mother-in-law, and in the end I just said "do whatever, and I'll just show up", which is what I did. With the way things have turned out thus far, I think it is clear your son and daughter-in-law have made the statement to their families that they are going to make their own decisions. If I were in your shoes, I would offer to chip in financially to the party, let it be whatever it's going to be, show up, have a good time with your son and new daughter-in-law, go home, and let them live happily ever after.
I have two daughters in their 30's, and have told both of them very clearly that I want them to have the wedding they want. If they should choose to elope, or get married in a wedding chapel in Pigeon Forge, I am good with that choice. I do not feel I HAVE to be there. I have always urged my daughters to live their lives, and refuse to do to them what was done to me.
Life is too short, and you have already let this eat at you for a month. That is wasted time you cannot get back. I urge you to be happy for them, and move forward.