I think she was hyped up from the sleepover.
If I'm 100% honest with myself, I can imagine doing exactly what you did.
However, putting on my perfect parent hat, I would ignore all the 'don't care' stuff, (With a 'ho hum' attitude - "whats that strange whiney noise in the background? never mind, isn't the sky pretty, what do you want for tea polkabrother?") and follow through with missing Brownies. (Using same attitude to deal with any 'missing Brownies' drama - too bad, so sad, I hear they found Richard III polkabrother!).
Re: homework - you could let her not do it/let her do it at last minute and take the consequences (might depend on how much you care about her doing her homework - are you prepared to let her take the hit - perhaps she could catch up on Brownie night?)
In future I would also enforce homework before sleepovers, if you think sleepovers are detrimental to her school work.
The whole idea is "your words are just washing over me and are completely pointless. See, I am a rock. I will always be there for you, but I'm not going to get involved with the drama when you are carrying on and I will follow through".
I think I would also allow her to calmly come up with a suggestion to solve the problem i.e. sleepovers are really exciting, but she has to do school work and be polite to her family and not flounce at pick up time - what does she thing she should do - let her own the solution as a problem solver and generally pleasant and polite person, who unfortunately acted out of character on this occasion.
If there is a problem of general rudeness and it has become a habit, I would draw up a list of expectations, and have some family wide penalties, that every one has to abide by, a bit like a swear box. I would try to be light hearted about it because I think, for children, bad manners are often a habit like nail biting, rather than something deliberately malicious.
Hope this helps!