Any advice on the internal structure for this? I was thinking of doing half foam, half real cake for the spheres, but can't decide if that would be enough to support each tier. I would put internal dowels in each sphere and then a central one through all of them. But since the central dowel won't go through the exact middle of each sphere would that still be ok?

The cake itself will be a dense one and will most likely use a stiff buttercream under the fondant. 

A central dowel will provide the support for all but the top ball, which is probably a dummy and very light weight.  A half sphere of styrofoam, on each ball, would be strong enough to support the cakes.  Your  plan sounds like a good one.

Another option would be to use RCT (RiceCrispy cereal mixed w/melted marshmallow) instead of styrofoam—then the whole thing would be edible 

If you decide to go with Rice Krispy Treats, I would recommend covering them with melted chocolate for added strength.

I would vote for foam half spheres for this project —

also if you want to continue the discussion please start a new thread — new members cannot “reply” to threads for a while — this is a stunning cake and I hope to talk about it some more with you — just those tiny title bits of flowers at each join is elegantly exquisite — hats off to that cake designer — lots of times cakes like that are all foam but I think at least part of that is edible — stunning