My main question is what do I do about the edges/borders of the cakes?
I plan on tucking the fondant under the cake board , but what do I do about the bottom?
I don't think anyone will actually bend down and look underneath each teir so do I just leave it alone?
Also I was going to stack them off centered and drive a center dowel through the middle, is that the right idea or am I asking for a disaster?

I think u are on the right track.

I think it looks great as is, but if you wanted to do a border around the overhanging edges and your going with the shell theme, I think just some of the tiny shells that are stuck on the cake placed around the overhung edges?

Hi i havly done one overhanging cake like this before and came up stuck at the same point. my solution was to cover the bottom of the cake boards with the same color fondant as the rest of the cake prior to attaching the actual cake so that when i put the cake on and applied fondant it looked like the entire cake was iced top to bottom.
And yes most definatley center dowel the entire cake it just makes transport (and your piece of mind) so much easier