HI,
Google second hand wedding dresses there are a few sites around and maybe you might find one that has been made for just a wedding the same as yours!
I agree inside and outside options either way because of the weather.
Thursday is fine, we would take a long weekend!!!
Maybe you should not invite your mother in law and tell her you did not want to inconveniance her because you know she would never attend a wedding on a Thursday because she told you that once