Hi Felix,
I think your routing plan makes sense, except for the fact that you wouldn't need any dry busses if you're using the pre-bus-send on your tracks. The track itself is providing the dry signal because the position of the track fader is determining the wet/dry mix. You would only need a dry bus if you were using the post-bus-sends, in which case the track fader is controlling the volume and not the wet/dry mix. Using the post-bus sends your wet/dry mix would be determined by how much you send to your wet bus versus your dry bus. Does that make sense?
I use the post-bus-send method to get my wet/dry mix because I like to keep the track fader as a volume controller, but there really isn't any substantial difference between pre-send and post-send in getting the right wet/dry mix.
I like the idea of using multiple Altiverb instances to get a complete sonic picture of the room. I'm going to have to experiment with that! [:)]
Brian