Has anyone ever used whipped frosting under fondant instead of buttercream?
No; I've heard it can't or shouldn't be done. Try searching Bettercream on here, seems like I saw that it won't work with fondant.
Real whipped cream will melt fondant. I don't know about all the bettercreams.
I tried pastry pride...disaster! The fondant was fine, but it was almost impossible to smooth, I had to chill it forever and try to smooth before it warmed up again...yikes..