Well, I've done the shelf/pvc thing before and I covered all of the edges with black pond foam to hide all the white bling.

That's how I had my old 120g. It was two tier, was nice, but I think yours could be better with your caves you got going on.

Just don't make any permanent decisions until you see mine, you might be able to incorporate my plastic tunnels and zip ties into your idea in some way. I think it would be cool to see your tunnels you are making as part of the foam background.