How do you cope with VERY different AFMA from center to corner on a lens (in this case the 50 L.
I have a 50 L that I have adusted to be spot on, seriously good, using centerpoint, but when I use an outer point the lens focuses 50-100 yards in front, 100% repeatble.
50-100 yards is a lot of front focus for a portrait lens that might normally be used at 10 ft, so you must be using it for landscape. I think it might benefit from repair at Canon, but, as you have discovered, its not a landscape lens.
I use the center to set AFMA. Lenses with a large field of curvature are not going to be in focus at both the center and edges unless you stop them down a lot. Lenses also focus differently at different distances, and for zooms, they focus differently at different focal lengths as well.
As long as the photographer knows the limitations of the equipment, he can make adjustments. You can manually focus to bring a item at the edge in focus and let the center be off, but most would not like the result.