[sdhotwn]

I had replied and then deleted a comment because I found that there are quite a few sources that define monocoque the way you interpreted it, which is not how I learned it in school - the outer skin has to be the stressed member is the simplistic explanation. I'd still call the M2 a unibody car, but seems like there is room for argument.

The diagrams and info would indicate that the difference is that all M2s have "something" assembled between the window/door frame arches to connect those portions of the body for rigidity. In a regular M2 it is riveted and glued steel beams, on the CS it is the carbon roof panel.