O.k., let me get this straight...your MIL wants to take you, her son, and grandchildren on a paid vacation to WDW at that ghastly time of year in July and you're upset because you're feeling manipulated and not the one in control?! Sounds like someone needs an reality check...and it's not your MIL!
Sorry, it might appear to you that she's trying to use her money to be the "Big Hero," but who are you afraid of losing face in front of...the children? I don't know any child who thinks less of a parent because they have to work to put food on the table. Admittedly, it might not be as fun as grandma's money and eccentricity, but just wait until you're the grandparent and want to do something for them.
And yes, I realize boundaries need to be established in any relationship, but is she living with you now and telling you or your husband-to-be what food to buy, what clothes to wear, etc? Just remember, how your husband-to-be treats his mother is exactly how he is going to be treating you for the rest of your life barring a major life-altering event. Also remember it is her money, not your inheritance quite yet, and be thankful she is willing to use it on you.
Well, so much for the harshness, I really do understand how you might feel as my mom can sometimes be that type of woman (though maybe not as rich or idle, and she has learned to suppress her grand generosity over the years since she has learned it really does make the son/daughter-in-laws a little uncomfortable); however, I can almost for sure tell you she is not doing it out of any sort of greed or manipulation. For that generation, especially if they did not grow up with alot of money, they do not want their heirs to miss out on the stuff they thought they were missing out on when they were growing up. Ergo, she really just wants to be around to watch her children and grandchildren enjoy a portion of their inheritance before she actually does pass on. In other words, she knows exactly what she is doing (unless, of course she's clinically insane and thinks she is the Queen Mum...no offense to our British DISers intended)
If I were Dr. Laura (thankfully I am not), I would tell you to go to her and apologize about your attitude and joyfully ask if there is anything you can do to help make this trip more memorable for her and her grandchildren. And, if she doesn't listen, tell her you really would like at least one day/night out of the trip that you can do something special for her since she has been so generous.
Rant and rave over, just a little sensitive in this area. Hope you and your family have a great time this July (but what's that, no planning sessions, no PS's, no scheduling every detail of the trip down to the last minute...oh my, she must be crazy!). But if she really wants be generous have your fiance' drop a hint that if she really enjoys Disney World that much maybe she can spring for a DVC membership!
-R