I battle humidity most of the year in Florida. My house is also old, drafty and temperatures inside reach 84 degrees regardless of my AC setting. Best advice is have a small dehumidifier by your work station, knead fondant on a marble slab really helps as the marble stays cool, work fast, adding glycerin to fondant will help if it is a dark color but the best thing to add to fondant in high humidity is Wilton fondant. It is the most firm of any fondant I’ve tried and although it tastes less great it always works in humidity if you can’t afford the massa that is made for high humidity. Adding just a little makes a big difference to the stability of fondant like satin ice which, for me, is very soft in the heat. Also delivering cakes in boxes sealed with dry ice inside makes a world of difference. Most venues do not have the internal humidity issues that my house has so the dry ice prevents sweating. As for ganache I make that more firm on hot days and have never had an issue with it melting off the cake. I avoid buttercream whenever possible except as a filling.