I went through the same "back and forth" as you are now experiencing about the 50L. I also bought the 85 1.2 II and felt at times. I wanted wider. I rented a 50L and was very impressed with the rental copy. It was relatively new, sharp(even at 1.2), no focus shift, beautiful bokeh and exceptional color and contrast. I was convinced and bought one from a large online retailer during the rebate sales. The copy I bought was unusable and I returned it for another copy. Same issues, so I returned the second copy. I really wanted this lens and would have bought the rental copy if I could.
I decided to take one more try and if the third copy was the same, I was going to put getting the 50L out of mind. Well, the last and final copy was the same, back focusing, but no focus shift. I talked to the local CPS service person and she suggested bringing it in for callibration which I did. The lens was callibrated and I couldn't be happier. Sharp at 1.2, no back focusing at all, sharp at all aperures and after hundreds of shots, no focus shift no matter what distance and what aperture. I'm very pleased and I have to say, it is now my most used lens.
The 50L is certainly a luv/hate relationship and you might have to go through what I did or have an understanding dealer with a good return and exchamge policy, but it's worth it.