I think your premise here is that the Nikon has the lens mount fitted to the outer shell
Not quite. The nikon mount seems to be connected directly to the internal magnesium chassis, while the Canon mount is connected directly to the composite mirrorbox which is then connected to the stainless steel chassis. Neither are directly connected to the outer shell, although my suspicion is that the outer shell on the Canon is giving some structural support to the lens mount based on the lip from the lens mount that sits flush against the magnesium shell.
whereas the Canon has it fitted to the internal chassis. I bet there is a reason for Canon to fix it to the chassis, possibly accuracy, after all the sensor is fitted to the chassis, not the outer shell. I would very much doubt that a D800 is built 'internally' to the same standard as a 4D, and my experience with the Nikon 'prosumer' grade of camera is that they are not inherently built to the same standard as the equivalent Canon, in fact I'm fairy convinced they are a cheaper unit, and the mount affixed directly to the chassis may be part of this.
Whether it's better or worse is really something that only their structural engineers can really tell you, as there can be very real advantages to composite parts (precision, toughness, thermal stability) other than manufacturing cost. As for them not being up to the same standard, I can believe that the internal sealing is probably superior on the more expensive bodies I'm not sure that the internal construction methods used will be all that different on the Nikon side. On the Canon side, the 1D bodies look similar to the D800 internally so an increase in build quality for the 7D would probably involve using a more unibody like construction for the internal chassis and/or better weather sealing.
According to the german brochure of EOS 5D Mark III
WARNING: german -> english (hopefully) made by a non-native speaker
"Gehäuseoberseite, Rück- und Frontabdeckung
der EOS 5D Mark III sind aus
einer robusten und leichten Magnesium-
=> Top plate, back plate and front part of the EOS 5D Mark IIIare made from a robust and light magnesium alloy.
der Kamera wurde für den harten Alltag
des Profis ausgelegt."
=> The bottom plate (?) made of steel is optimized for professional use.
"Innen wird ein Aluminiumchassis
von geformten Kunststoffelementen
geschützt – das schafft ein sicheres
Gefühl von Zuverlässigkeit."
=> Inside an aluminum chassis is protected by moulded elements made from
plastic - this gives a feeling of reliability.
I think your conclusions are right, that the EF mount is mounted via a plastic (of high quality) to the inner frame - just change from steel to aluminum for the inner frame. As I remember the EOS 20D has a steel frame - perhaps steel applies to the 5D mark I ?
The second red part of your comment:
WARNING: I am no engineer just a physicist. But basically you are right: Putting the mount, the outer shell and the inner frame with the CMOS sensor together has a strong disadvantage. Each hit against lens or shell accelerates the sensor and AF components directly and transports horrible forces.
A interlayer of plastics which are deformed temporarily decreases acceleration by e.g. a factor of 10 and reduces the forces on components by the same factor.
The plastic components are made from polycarbonate (PC is the acronym which is visible in one of the photos) which is used for bullet proof windows or CD ROMs where high strength and "form stability" is essential. I just would not have problems with a well made PC shell - it absorbs more energy than a magnesium alloy shell during a hit ...