Can I use cake flour?

For brownies, it shouldn't matter if you swap in cake flour, since they are meant to be soft and they sort of melt all together in a pan (as opposed to relying on a more individual structure, such as crisp cookies). The extra egg will keep them a little firmer -- good idea!

I can't see why you couldn't. They'll be more tender, so your idea of the additional egg is a good one.