Jessica,
I understand how you feel that your in-laws have hijacked your wedding and I think it is wrong of them to turn a deaf ear to your feelings. You sound really exhausted by the process and the coming demands of that weekend.
That said, I really feel that this is a conflict of persective, not intent. Your in-laws honestly believe this “how it's supposed to be” and they probably feel rude excluding the many out of town guest from their dinner. Some families are like this. Likewise, you have strong feelings on “how it's supposed to be” and you seem to feel that they are trying to compete with you in some way.
I do think you should try to humor their plans for this party, with a few caveats. Please allow me to spin this party in another light for you:
1. No fancy dinner can steal the meaningful celebration of your wedding day. Your wedding is not a movie with a suprise ending that will be spoiled if you see cousin Millie or his Uncle Bob the night before. Everyone will still gasp when you appear in dress and veil. Also, you won't notice those people lining the aisle when you walk in the church doors anyway…they all blur together.
2.You wedding day will whirl by so fast that you will only really get to talk to guests for a few moments each. This party gives you the opportunity to really enjoy their company, especially considering that not everyone will attend the Sunday brunch.
3. I absolutely agree that you need to unwind and give thanks, and this does, sometimes happen at the rehersal dinner. I don't know how involved your in-laws have been in the planning process, but traditionally this is the one aspect that they have any control over and I do think you should appreciate that they love you so much that they want to invent money in ANOTHER big party in your honor. It's generous, however blind to your feelings.
A large party does not negate thanks, but if you require intimacy, wel, thanks also needs no occasion. Fiind the time earlier to express your gratitude individually to each person who has been instrumental.
Unlike your wedding, you are not the hosts of this party. That lets you off the hook and I think a good compromise would be to arrange with in-laws to stay through dinner, toasts and A drink. Then you, bridesmaids, groom and groomsmen are sent off early in grand fashion by the relatives for your respective accomodations. Maybe you all meet up in the hotel bar or at home for one last goodnight, hugs and laughs before bed?
By no means am I suggestion you sacrifice your happiness or repress feelings of discomfort. I am suggesting that you try to compromise so that their generosity is not insulted and you are able to relax a little before your wedding.
Also, what does your groom have to say about all this? You mentioned yourself and your parents…but what does he have to say?
Much luck. I hope your day is beautiful and joyful!