ACan you pipe a buttercream border & put buttercream decorations on a whipped cream iced cake?

I think you can have a small border and maybe very small buttercream decorations. I read a post here on CC about buttercream rosettes that fell off of a whipped cream iced cake. I think the whipped frosting was called Pastry Pride.
 

AThe reason I was trying to do it this way is because I'm using a carton of bettercream whipped cream to frost a sheet cake. I was told the sheet cake will use the whole carton. I didn't want to open another carton just for a border & I already have plenty of buttercream. I'm just not sure what would happen, if the buttercream will make the whipped cream melt or what....

The problem is the buttercream will be much heavier than the whipped cream and gravity will take effect and could cause you some major issues. Personally, I would not take the chance,

AYes you can. Just make sure your borders are on the inside of the edge and not perched right on the corner as is the norm.