Wonderland Gal, Best Wishes on your upcoming nuptials! What a wonderful experience to pledge your life to your beloved on a beautiful autumn afternoon, and to do it at Disney No Less!
You are getting married at Sea Breeze Point which is a beautiful and romantic location. I suggest since you are looking to feed the 20+ guests without breaking the budget that you consider Captain's Grille at the Yacht Club. Your guest would not need to use a day's admission to a theme park, the logistics of getting everyone to the restaurant would be easy, and the price is reasonable for nice luncheon food. The clam chowder is lovely, my son is fond of their fish and chips, my hubby likes the lobster roll, and the gelato is a favorite of us all.
If you don't mind everyone grabbing a bus or taking a car to the luncheon, then Raglan Road is a great choice. They have plenty of space and great food.
As to your bolded comments about a mother and sister in law, darling, unless they are paying for everything, then their imput should have no more weight than any other guest at the wedding.
If you are ready to be a wife, then you are ready to learn how to deal with difficult people effectively. First, really examine yourself and ask yourself if you have been as open, fair, and accomodating as reason would dicatate, then make sure you and your intended are on the same page, and then put your plan into action. Smile, nod, be cordial, and keep on doing what you are doing.
The best gift you can give your marriage is to establish healthy boundaries around your family unit, which for now only includes you and your husband.
Wherever you end up there really are only four options, and none of them are which restaurant you will use:
You can let "them" have their way and complain, moan, and gripe about it endlessly, starting your very own bitterness boulder which you will drag with you for the rest of your life.
You can let them have their way because you decide it isn't that big a deal where you eat after your wedding as long as you are Mrs. Most Wonderful Man in the World. You can decide that if eating at XYZ makes them happy, then so be it.
You can pick a restaurant of your own choosing and then complain and harp and yowl about how difficult the whole process was and how "they fought me and don't like me and are so picky and HORRIBLE" for the rest of your marriage.
You can pick a restaurant and graciously let everyone know what you have decided. Then you can sit back and allow anyone who wants to act like a horse's business end to make a public spectable of themselves while you remain calm, ladylike, and serene.
I am giving you this advice after a couple of decades married to a wonderful husband who was raised by a delightful father-in-law and a terror of a mother-in-law. After a decade of tears, frustration, and pain I finally came to the conclusion that there was nothing I could do to make her happy, because being unhappy is her hobby in life.
So, the best I could do is to be ethical, kind, open, and love her son and grandchildren. I do that by making my marriage strong to my hubby and by treating her with respect that she has not earned, but I will give her anyway because I LOVE her son so much. I have decided to treat her the way I want my children and their future spouses to treat me, not the way she deserves to be treated. Because, frankly my marriage, my children, and my dignity are just a whole lot more important than she is.
In the end, she makes a spectacle of herself and the rest of extended family and friends have decided that I am wonderful and are convinced that my husband married "up." Most importantly, my wonderful husband and children, who have a front row seat, are proud of me, love me, and think I hung the moon.
Edited to add: Remember, you will NOT be allowed into the parks in your wedding attire. If you decide to eat in a park, make sure to plan enough time to change before you head to lunch.
it's a delicious formal meal and the staff is top notch. it would be close, and it wouldn't force anyone to use a day of their tickets for that day. 
Seriously, I have no idea but its your day and you should do what you like best. Have a wonderful day and don't use up all the disneymoon magic before I get there 
