I like your fifth idea! How about both the 7D and 70D both being mirrorless cameras with shortened flange distances and Canon introducing a 1-1/3 stop Speed Booster-like adapter? The FF cameras could keep the standard flange distance and mirrors for photographic Neanderthals. The M-mount might have been introduced, half-heartedly, solely to allow a reduction in camera size.
Please, no. With my "Neanderthal" EF-S camera, I can shoot pictures in a theater and not annoy everyone with my bright screen. I can look away from my optical viewfinder and enjoy the show without a burned-in square where the screen killed my dark adaptation.
I mean, I could maybe see it as a second body (sharing EF and EF-S lenses with my main camera), just for size reasons, but then I'd have to carry around two cameras just in case I wanted something lighter around my neck, which sort of defeats the purpose of buying something lighter to begin with.
Besides, if I cared about camera size, I'd be shooting with a point-and-shoot (or, for that matter, my iPhone). Whether you go EF-S or EF-M, the vast majority of the weight is the glass, and comparable lenses require very nearly the same amount of glass whether they're designed for EF-S or EF-M—the EF-M 18-55 lens is actually slightly heavier
than the equivalent EF-S lens—so I just don't see any real benefit of EF-M, much less a benefit so significant that it would make up for the loss of an OVF.