I have a cake, buttercream with swags, due for this Saturday and it's supposed to get really hot and humid. Should I add something to the mmf to make it more sturdy and to help the swags lay right? TIA

My only comment is "Good Luck!" Buttercream with fondant swags on hot day?

I spoke with the bride and she said to go with fondant on the whole cake and they are going to keep the cake inside the building so it doesnt melt, yay!

that's great news!
It will really make your day a lot easier(and less stressful!)
Jodi

You could add CMC to the fondant before you roll it out. It will make it firmer, somewhere between normal fondant and gumpaste.