Fuck the heels thing. That's outrageous (I know, I know, they're good people, dear friends ... but that IS outrageous.)
As is the hats/fascinators thing. Not everyone suits them and not everyone will already own one or want to wear them again; they can't expect people to spend money on something they'll wear once.
I'd be tempted to wear a dress with a belt, so no one can tell if it was a dress or a matching skirt and top. 
I know that's not helpful. But lots of other people have posted helpful stuff.