Agree with PP that she thinks you weren't going to ask her to be a bridesmaid.
I think she's jumped the gun a bit, but it's understandable because you didn't ask her when you got engaged or make some enquiries about some dates she was free. TBF, that close to her own wedding, she might have been away on honeymoon, lots of people delay it a bit now.
It is unfortunate that she is a demanding bride, but I think it might be better to treat that as a separate issue, otherwise this will just blow up and you will both be hurt. Think it's best to proceed as if this is all a big misunderstanding really. Unless you are happy about losing the friendship.
Used to organise events professionally and its good form to find out when the venue is free and the key players are free, then decide on the best fit all round from there. Precisely because it avoids this kind of thing and especially if you are arranging something at short notice/in a peak period. It also gives people a chance to bow out gracefully if they see you have your heart set on a date they can't make, without themselves feeling hurt that they don't matter enough for the date to be moved.
And some people really are sticklers for going with the first invitation that they accept. It can sometimes go against the grain a bit for all concerned, but there is also something reassuring in it- after all, that kind of person is unlikely to dump you for a better offer at another time.
Just as it is also really bad form to invite just one member of a couple to a wedding, also it's not really fair to expect her and her new husband to go their separate ways to different weddings, especially when they are so newly married. It's the kind of thing you can accept if someone offers to do it, but you cannot expect someone to do.
And yes, there is a hierarchy of friendship above said as someone ab, and a spouse comes at the top of that tree. So if she promised him she would go, she cannot break her promise to him to favour you, irregardless of her promise to the other bride and groom. She likes him best in all the world, that's why she is standing up and marrying him.
Agree that some things are more important than ettiquette. However, correctly used, protocol and etiquette is a way of avoiding these situations. You jumped at a good slot at a venue rather than considering the people you wanted there on the day. And you didn't consult the people you wanted to be there in advance or even give them notice you wanted them to play a special role. So some of this is on you to sort out.
You are right that etiquette can be used as a weapon, but it could very well be she was hurt as she thinks you don't want her as a bridesmaid, and is using those excuses to defend herself.