Now that I am back at a real computer and can type more easily ...
Wwelcome, Lynet!
Rose’s Mousseline stands up best of all the other buttercreams to heat and humidity, and it tastes fabulous! Of course, any buttercream in direct light will become problematic, but if the cake is held at cool room temperature until serving—i.e., it’s not out in the heat on display for hours before cutting—it should do perfectly well.
There are mousselinles in both RHC (Rose’s Heavenly Cakes) and in TCB (the Cake Bible).
Good luck!
—ak