I have been under the impression that full frame compact (mirrorless compact) cameras are hard to do because of the required distance required between the sensor and the optics. ... I think all the full-frame, compact, cameras to date have had fixed-focal-length lenses that were not removable and were in the wide end of the normal range (I am guessing from memory here, but I think around 35mm). My guess is that this is because around the 35mm focal length a lens can still project a wide enough image circle while remaining small and high quality.
I partly agree. So long as the attraction of "compact system cameras" is based in good part on compactness, then the lenses need to be compact too, and scaling up to 35mm format ruins that, unless one sacrifices the advantages in low light performance and shallow DOF that are so often given as reasons for using a bigger format. For example, to fully keep the low light performance and DOF advantages require keeping the minimum f-stops of lenses about the same while scaling up focal lengths by a factor of two from Micro Four Thirds to 35mm format, for a factor of eight in the volume of lens elements and such.
That is probably why all the recent efforts to put a big sensor in a small body (various Sigma models, the Sony RX-1) have not offered much in the way of longer than normal focal lengths, or interchangebale lenses, or zoom lenses, or even the primes of low minimum f-stops.
Instead it is all prime lenses, and none with particularly low minimum f-stops. One fundamental limit on how small a lens can be is its maximum entrance pupil diameter, or effective aperture diameter: focal length divided by minimum aperture ratio. And what we have in "big sensor compacts" is things like:
35mm/2 = 17.5mm for the RX-1
50mm/2.8 = 18mm for the Sigma DP3
24.2mm/2.8 = 8.6mm for the DP2
None close to even the 30mm of a modest 50mm, f/1.7 standard lens.
As a result, in terms of the much-touted low-light handling and shallow DOF options of a larger format, none of these delivers any advantage over what APS-C format compact cameras can do with f/1.4 to f/2 prime lenses.
Beyond this narrow zone of not particularly fast wide to normal prime lenses, removing the mirror and OVF does not contribute much reduction in the size and weight of a 35mm format system, because lens bulk takes over. I am not sure how much market pressure there is for "non-compact mirrorless systems", but my guess is that Sony might soon test the market on that count!