Two cake issues Suddenly!

Hi everyone!

I have two problems happening at once to me today as I am working on weekend cakes:

1.) A square tiered wedding cake, where I will be stacking one layer on top of another directly.... a)my buttercream icing is starting to crack and split on top, I use the wilton wedding recipe and b.)I need to cover a square in fondant on this and I can't for the life of me figure out how to do the corners because everytime I roll out the fondant to the proper depth it splits and cracks when I am putting it on the cake...I am using wilton's fondant too

Thanks for your help!!!