I'd personally wear a hat or not go, it's clearly something very important to her for some reason, plus I like hats, any excuse for a hat... but I totally get not liking being told what to wear as well x
Yes. You could wear a fascinator. There are some pretty little ones available and they are not expensive. A colour in it to match the dress you'll be wearing or a colour to match your shoes.
I don't really mind wearing it but I just don't like her telling me how to dress, as long as we look decent and well turned out (which of course we will) and as Ummm says I don't think she would stop me going if I didn't wear it.
I know I'll end up towing the line and wearing one just to please her.
It's not really like being told what to wear in a despotic sense though is it? I can see no difference in this to invites that say morning dress, wear pink, no black at funerals, no jeans or trainers, come in 70s gear yada yada.... I expect she wants a traditional smart wedding.