Hi @Laceandfrills.
I'm glad you've found a resolution. It's difficult, but I'm glad your future DSD will be involved in some way. It's also right and appropriate that she should be with her father. In terms of who enters in what order, though, I'd say that this doesn't indicate a hierarchy of importance where family is concerned. I lived a few years in the US - there, the bridal party comes in first (flower girls followed by bridesmaids) and the bride, as the most important person, comes last, as the Queen always did.
I know leaving a 7-YO old out of the bridesmaid experience (every little girl wants to be a 'princess', ugh!) sounds callous at first glance. But from the other perspective it's not a reasonable expectation at all that a bride should have the caring responsibility for a neuro-diverse child she doesn't even know (we all know that's what would happen were she in the bride's personal party) on her wedding day. Her own relatives, her father and grandma, should be falling over themselves to ensure she's okay and I'd have to question why her granny at least is not a taker. A wedding is not the best of circumstances for a stepmum to get to know her DSD better: you don't know how she reacts to stimulus for one thing; mixed gatherings can often be tortuous for neuro-diverse people. I consider your future MiL's behaviour an absolute let-down in this respect.
I can see why you've been hesitant, as far as marriage is concerned. MiL sounds both unsupportive and dictatorial. Your partner sounds as though he needs to fight harder in the interests and wellbeing of his child. This is less 'bridezilla' or 'wicked stepmother' - we're not in a ladybird book here - than a cool-headed, practical set of considerations as to how this will actually work in practice.
You are not walking into the easiest of situations here OP. I hope things work out for you. At the very least, in your shoes I'd be wanting to keep your impossible MiL safely at a distance once the wedding is over with.
Hope it all goes off well, and if this doesn't sound too hollow, congratulations! 