Hmmm....that's a tough one. I feel really bad for your daughter :( Being involved in Girl Scouts, I totally understand the dilemma this puts on the Host Mom/GS Leader. We are not supposed to exclude any of the girls so I know where she is coming from. However, if she were in our troop I would have no problem telling her Mom to get her kid under control or I will. I will not sit idly by while someone is being bullied.
There are times when I say it is very important for kids to learn to navigate through life and deal with things on their own. However, I do not feel this is one of those times. She is 10 years old and Jane pretty much bullied her the entire night last year and makes her miserable every time they are together. I'm not reading anything in your post that makes me think Jane won't opt for a repeat performance of last year and frankly how many hours of Jane's bs should a 10 y/o have to take?
This is such a rough age for your daughter to begin with and to have to have all this drama dumped on her is uncalled for. I think you've handled all of this wonderfully. The only other solution I can see is if you're open to it let your daughter have a sleep over in the near future inviting all of the other girls that she does get along with and exclude Jane the Pain. (Yes, I know I just called her a name...but I just couldn't help myself...I hate a bully.)
There is no need to wait for her birthday or other special occasion. You/she could host a good old fashioned slumber party with only the nice girls. You could do a "cheer" themed party, kids cooking theme (have them make their own pizzas for dinner), jewelry making, spa day, or even movie night. I'm sure if you google it there are a ton of age appropriate themes for slumber parties.
That way your daughter can still enjoy the pleasure of the other girls company without the stress of dealing with Jane's drama. Yes, she will still be a bit sad about not attending the birthday party but if she knows she will soon host her own party with the same girls in a few weeks that should lesson the blow.
You are also teaching your daughter that she is entitled to be treated properly and she does not have to put up with anything less. Her physical and emotional well being are more important than anything else on this earth. The best lesson I hope to teach my girls is their own value and self worth. If you know, and I mean really believe in your heart that you are a wonderful, intelligent, compassionate, loving, beautiful soul......then nothing anyone can ever say or do to you will EVER make you doubt yourself. You will be so confident in who you are as a human being that you will not tolerate anyone disrespecting you.
Sadly, we can't fix all their boo boo's and heart aches but they are only this young once and well gosh darn it, I'll do what I can to patch them up when/where needed. Honest, even at 10 it's ok if Mommy makes it all better on occasion ;D Hugs to you and your daughter.
Peace and Blessings,
T. B