Op come on... I've read every post and I'd say usually I'd be pretty laid back around wedding stuff and roll my eyes at anything bridezillaish but you are coming across like a nightmare in law here.
What is your parents contribution to their wedding anything to do with anything? In what way does your mum contributing entitle you to do what you want? You have not contributed anything and I absolutely cannot stand people who feel that gifting someone money buys control over those people. Horrible attitude for your mum to have and particularly for you to have since it's not been your money. I would never expect to speak to suppliers, dictate food options or venue etc if I were to give my son and his partner money to help them get married. That's massively OTT.
Calling it a "wedding event" rather than a wedding is also really telling. You're minimising the importance of this to them. It's not your "mums wedding event" it is your brother and his wife's wedding. Honestly if I were in their shoes I'd be handing your mum her money back, eloping and putting serious distance in with the two of you.
The fiance saw a dress she felt was too white to pass as a guest dress. Which is fair enough each to their own. Your mum felt your dress fell into the category so gave you a heads up. Why are you centering yourself in your brothers wedding? Your dress is a cream/ white/ ivory dress with a bit of coloured detailing. If you cannot see that then I'd make the time for a trip to specsavers. Whether or not you feel its appropriate is irrelevant, you know it doesn't fit with their dress code which is why you're painting her badly to try and curry agreement on here.
You've also criticised her for not "having the balls to tell you in person". Why the unnecessary aggro towards her if she's not even seen your dress and therfore has no reason to tell you anything in person? Calling her nosy for asking if you have a dress yet which is a perfectly normal thing to ask about a sil. Why are you trying to present her negatively? It sounds like you are trying to set this up in a way that people agree with you rather than just in a way that's not overly outing. And op all of that is your issue and nothing to do with her.
If you give a hoot about your brother and if you can find it within yourself to acknowledge that their wedding is a really big deal to them so it's fair for them to want to do it the way they want to do it, then order a new dress from next online wear that one somewhere else or return it and put your best foot forward with your new sil.
I think your behaviour, your attitude and the way you are talking about your mums money in particular is appalling. I feel genuinely sorry for your sil.