I'm liking that the ACA master himself is getting into rendering inside ACA and I am looking forward to learning further how to get the best out of the system. Archidigm has a new post of a very nice internal render on his front cover. My feeble attempts at internal renderings are humbling but I would like to get there. I notice his furniture featured which is available as content at a very good price.
He's also written another article on rendering materials across X-Ref's and some of the problems you have. He notes that RMats work a little differently than other objects. When you bring x-ref's together for the purpose of rendering a model, all the materials are brought into the file and become part of that file. So you can then alter the materials in the host file unlike for e.g. a wall style which needs to be edited in the original file, saved and reloaded. Unload the xref and the materials remain (unless purged). Update the material in the original file and the material is NOT updated in the host file unless it's deleted and then reloaded by reloading the xref. These complications are why I like working in a 1 file approach. Recently pulled my 1 file model into another file just to do the render (as it was after doing the CD's) and I certainly confirmed the simplicity of my approach as I encountered the problems Odin addresses. However if you require a team to work on a project together then splitting the file using the Project Navigator can be a good idea despite the drawbacks or you may just like breaking the project into manageable pieces.
I like his pick of how many steps it takes to swing a door in Revit. Not knocking Revit overall of course but it's nice to see that ACA still has the ascendency in some areas.
Anyway just thought I'd post to let you know I am still alive. Barely breathing but still kicking.