I'm not sure I have put this in the correct forum.
I just took my first 16" square cake out of the pan and as I was trying to get the sides to let go of the pan, I cut the bottom edge of the cake on three sides. So, now the top of the cake overhanges the bottom by as much as an inch in some places. My question is this: Can I use the scraps from this cake and mix them with buttercream and fill in those areas and cover it with fondant, or do I just need to make the whole thing again? It will be the bottom of a 5 tier cake with all the appropriate support structure, and I can't cut the cake down because it will be too small to hold the 14" round that will go on top of it.

You may do that or I would just ice it with ganache.