Need help with an alteration please?

[Katymac]

DD bought a sort of neoprene bodycon dress & it's too big

I have fitted it & I think 2 tiny tucks under her arm will really help - but the back is too wide until the waist & I think I need to take a dart in (which I have pinned while on her)

BUT the place the dart needs to be there is a false (fully functional) zip - so could I resew the zip and cut away the material in a V shape so it actually does stuff?

Maybe?