Very interesting ... And somewhat concerning read. I would love for someone to make a post on how to test for this!! We amateurs rarely get as technical as adjusting focus per lens! ___________________________________________________________
Reality is merely an illusion, albeit a very persistent one. - A. Einstein
One great thing about Live View cameras is that you have 2 completely independent autofocus systems. This offers the possibility to use one as a diagnostic tool for the other.
The conventional (phase detect) AF sits in the top of the prism and focuses from the image reflected off the mirror. It's critical that the light path from the mount, off the mirror, and up into the phase detect sensor is exactly the same distance as the mount to the sensor, otherwise this AF will not operate properly. Most cameras have an offset screw to fine-tune the position of the mirror at rest, and misalignment of this screw, skewing the distance light travels to the AF sensor, is the typical cause of AF problems.
With Live View the camera now offers a contrast detect AF that uses data directly off the sensor. Because this is exactly the same image recorded during exposure, there is a lot less that can go wrong.
Bear in mind that contrast detect AF (LiveView) is optimized for accuracy, while phase detect AF (traditional) is optimized for speed. Contrast detect AF will therefore give more consistent results, and by its nature phase detect, even when working properly, will always be a little bit hit-or-miss. But, if phase detect AF is consistently off and contrast detect AF is always on, that's a good indicator there's no problem with the lens, mount, or sensor. Comparing the two is how you can isolate a problem to exist (or not) in the phase detect AF system.
>How is it possible that the AF sensor is causing "a soft >left side".
I don't have the problem but I read that as saying "if the AF point is on the left side the focus is soft, if the AF point is not the focus is sharp" meaning that the AF sensor was not in some fashion parallel to the primary sensor plane.