Hmmm, talk about being stuck between a rock and a hard place. I think that depending on your relationship with your FMIL I would call her directly. I would tell her that you don't want to hurt her feelings in anyway, but that you and your FI prefer to save all of the celebrating for Mexico and that also, you don't want to exclude your family members from anything that is meant to celebrate your marriage. Ask her to put herself in your parents shoes and she would probably understand- I doubt she would be happy if your parents got to celebrate with you and FI and she wasn't able to attend. Talking to her yourself will give you a better idea of how she's feeling since your FI sounds like mine and doesn't give enough detail about stuff like that. I would probably apologize for agreeing, but say that as much as you appreciate the offer and think it would be a great time, it just doesn't fit with your plan to keep it discreet.
I feel your pain on this one- we are having a hard time deciding what to do about our legal at home. On the one hand we would like our parents to be there and on the other we think they will be like- what the heck are we going to Mexico for if you're already married?
Let us know what happens.