It really sounds as though you are level headed about your money and your fiancé's money. But, to suggest or ask your future MIL to chip in her fair share is very tacky. Besides, then you will not have the wedding YOU want, but the wedding SHE wants for you and her son. That is not going to make you happy in the long run. DO NOT give up all the power and planning of your own wedding.
You two should probably elope and take your MIL up on her offer for a honeymoon. Then, when you get back, have a nice Sunday afternoon cocktail party with hor' devours.
I was at a lovely back yard cocktail party with about 85 guests, open bar with bar tender, delicious finger foods and an ipod music playlist, rented stemware, some borrowed seating from neighbors, appetizer plates, dollar store napkins, a teenager to time the oven food and plate it.......all for $1500.00. See, you take away the word "wedding" and you have just saved thousands upon thousands of dollars.