I assume you have found somebody to do your cake. If not, I urge you to find them NOW.
They will tell you that as long as the room is air conditioned, you need not worry about melting, and your options with buttercream are plentiful.
Contrary to what you've stated, the only thing that could pose a problem is an overlay of fondant or marzipan, because the humidity at that time of year could cause condensation to build up on its surface.
Good luck. Gotta run.