Buttercream does not like heat or humidity. If you are warm and sweaty, then your buttercream is probably warm and sweaty too. A fondant covering would protect it some what. I am sure you can imagine the temperature it would take to make butter start to get soft and I think that would be a good indicator as to when your icing would start getting soft under fondant.
I have found that swapping out some of the meringue powder, with equal measure corn starch, helps tremendously.
I honestly don't know if you can kill the super sweet sugar taste in buttercream. Sometimes a little fresh lemon juice cuts sweetness. The lemon would add to the flavor of the buttercream. I would adjust it into the liquid amount you are adding to the buttercream, but probably no more then a tablespoon.
My buttercream icing (and I do not stray from my favorite recipe) is half butter, half shortening. Most of my cakes are covered with buttercream, including wedding cakes. I live on the Outer Banks of North Carolina where the temperatures have been in the high 90's and the humidity is almost always near 80%. I work on my cakes in the air conditioning, and I suggest my customers keep their cakes air conditioned, not refrigerated. The humidity is so bad here, that refrigerating cakes and then bring them out causes condensation.
I hope this helps,