I don't know why I agreed to this. The MOB baked the cake, I am decorating it. (the groom is my DH's cousin)

Anyway it is a heavy fruit cake. And I mean heavy. They want 2 tiers, but then MOB decided the cakes she had baked were too shallow so she made another huge square. So I was presented with two small cakes - 7 and 6", and a 12". I cut the large one into pieces, so each tier is made up of two layers, one layer being two rectangles. Each tier is a square, about 4" thick now. With me so far?

Ok so I think I can get the shape right. The problem is how to glue them together so nothing shifts. I am also concerned about how the cakes will cut, I don't want them falling apart. There were already a few cracks showing through the centres.
Should I use a softened marzipan layer between them, or try and stick them with apricot jam (jelly?) Then go with fondant over the top?

I have made a lot of cakes in my time but no fruit cakes, so this is all new to me.

Any help would be much appreciated!