Your’re almost there… I think this is what you want to do.
Make titles that say camA (drop), camB (drop), etc. Then create mixdowns and use those “clips” as place holders for the cameras that drop out. Every time a camera drops out, the position in the quad/nine split is unchanged. Not only that, it is pretty clear to the editor what is going on.
Hope that helps and thanks Nate (AE) for the idea.