Since most lens-to-camera connections don't have an issue with light leaks, what would make you concerned that an additional connection between the lens and camera would?
The connection on the front of the adapter is identical to the lens/camera connection when using the lens with a Canon camera. The connection on the back of the adapter is identical to the lens/camera connection when using an E-mount lens with the α7R III.
Usually the primary concerns of using adapted lenses are:
- Communication issues between the camera and lens, particularly with regard to autofocus and aperture control
- Accurate alignment between the optical axis of the lens and the sensor in the camera. Adding an additional interface multiplies the error introduced by manufacturing tolerances of the mating mechanisms.
In the case of using the Metabones V, the Sony camera should have no issues communicating with the electronically controlled aperture of any of Canon's manual-focus-only TS-E lenses. Since you're presumably going to be using the tilting and shifting capabilities of the the Canon TS-E lenses, the alignment issue is also pretty much moot.
I can't say if there are any problems with light leakage using the Metabones V. But if there are, it would be fairly easy to mitigate in the same way one can mitigate light leaks when using filter holders for long exposures.
Anything from gaffer's tape to shielding the connections between the lens/adapter/camera with a hood or opaque cloth should do just fine.