Hi there.
I am sooo very sorry to hear about your situation. Why something that is supposed to be a happy & joyous occasion can make tensions flare and cause people to act immaturely or out of character is beyond me? I feel so bad for you. We had a basically care free wedding. My husband's father is dead so the only parent there to be involved in our wedding was his mother. She can definetly be a handleful. She did have nice things to say about the wedding, but she did not make a big fuss and she didn't even tell me that i looked nice. We only had 15 people attend and my parents paid for a little more than 1/2. My mother in law took care of the rehearsal dinner with no problems and actually seemed to enjoy it (we went to the Hoop De Doo). As a side note to anyone, i would DEFINETLY recommend the hoop de doo for a rehearsal dinner. It was a good choice for us because it wasn't formal and it was fun and relaxing. I am sure alot of people would like a more formal dinner but we were having a formal dinner the next night for the wedding, lets relax and have some fun.
Anyways, i digress...my mother in law finds some way constantly to say the wrong thing to me. She came to look at the pictures after the wedding for example, and did not like the pictures of her. To give you some background, she almost begged me to have her there for pictures with her son before the wedding. So, i arranged it gladly. Then she comes, sees the pictures, and tells me she hates having her photo taken, she doesn't take a nice picture....blah blah blah... I was so mad. That is just one small example, she was complaining the wedding day too...she is an absolute chronic complainer. I can't stand it personally, but her heart seems to mostly be in the right place so who am i to say something. However, sometimes, you have to speak up for yourself. I, like you it seems, are a nice person who takes everyones crap and hates confrontation. This is not a good way to be. My sister-in-law caused me pre-wedding problems too which i won't get into but i have let her talk to me like dirt for 3 years now. She is a very mean and nasty person and just like everyone else, i let her talk to me however she wants. NOT ANYMORE. That started before the wedding, and i am glad i spoke up to her. This did not give her any further opportunity to ruin my wedding.
So, what i am rambling about is...as much as you want to keep peace and be the bigger person, i don't know if that should always be the position you take with people who are rude and telling you how to do YOUR wedding. As selfish a thing as it may sound, it is yours & your fiance's day. Yes, the parents are extremely important, but not at the cost of the bride & groom. Nothing you are asking sounds unreasonable. We had a VERY limited budget and i have a big family. Only 15 people came. I have more cousins on my mothers side than that and most of them are married and some with kids too. The fact that you and your family are planning to offer to pay for people SHE wants to invite should be enough. 300 is ALOT of people. We were happy to have a small wedding because i think it eliminated some problems we could have run into as well. I would have liked to invite my family and some of his (he has a small family who doesn't really spend time together), but we couldn't and that aleviated some the stress that unfortunetly you now are going thru.
I wish i could be more help but having been through things like you are going through now all i can say is. Be kind, but firm. Ultimately, if they aren't paying, what can they do. The fact that there is talking-back to you and your future hubby about things they aren't paying for sounds rude to me. I feel for you and i know you don't want problems, but they have been presented to you...you didn't start them. You have every right to kindly speak up for yourself and i am sure your fiance will back you up.
Keep us posted, I do hope you get your dream wedding, and all the wonderful people here at the DIS will do what they can to help.
Take Care of Yourself & don't ever doubt yourself, if it feels right to you...
IT'S THE RIGHT THING!!
Have A Disney Day!!!