My understanding is that the rear body on the coupe is formed from SMC (Sheet Molding Compound). Your information on the top panel is new to me. I had always assumed that the entire top panel was made of magnesium.On coupes the roof is magnesium framed (and IIRC SMC in the panel?) and the rear body around the hatch is non-ferrous - not sure exactly what it is.
There are several aluminum structures supporting the SMC rear top, all mentioned in the quote from J. D. Powers in my previous message.I rely on the release announcements from GM for that two component construction of the roof panel. It is hard to tell as there is an inner liner, so you can't really get at the centre of the top to check it out. Anything is possible - early announcements mention aluminum rather than magnesium, which is clearly wrong.
You are probably right about the SMC rear panels - again, I couldn't find a specific reference to that.