tallyhoo - your friend enjoyed being "better" than you, by the sound of it - she liked to put you down to make herself feel better about herself.
She didn't want her work friends to actually like you - she wanted you there to make her look better, and instead, you turned out to get on well with those friends.
I agree it is like a reverse "wendy" story, except that you didn't do it deliberately. But your friend was feeling left out because she could see that you were getting on with her other friends, maybe better than she does - and now you're all being punished in a childish fashion because she can't handle the concept of people being friends with others without her as the contact.
If the other girls are too scared of her to stay your friend without her, then I would bin them off. If they were genuinely interested in staying friends with you then they would have found a way to do it, not bow down to her tantrums.
I have been in your position once, except it was just over a weekend - went to stay with a schoolfriend at her university shared house, and got on well with all her housemates. She ended up going to bed early, I stayed up chatting with them, and next day she was in a huff with me. After I left, we had one phonecall and she never spoke to me again after that - and never really explained why it was so bad that I got on with her housemates. I wasn't excluding her at any point, she was the one who took herself off out of it.
Go and find some other friends.
ANd in all honesty, I'd reclaim your money for the October trip and not go - it will be a VERY uncomfortable trip away, if they even "let" you come - there are ways they could stop you.