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This will leave him with part of the view being blacked out. So take your circle and on the left side of your circle, draw a sliver of a cresent moon from the top crosshair, following the curve of your circle, to the bottom crosshair. This cresent moon blackout you've drawn is what it might look like for someone who the gun/scope is not set up for. Now since he's not in the right place (seeing the whole scope) then he has now changed the place where the crosshairs are pointing. And I believe if he had the left blacked out a little, and put the crosshairs on target, I believe his shot will fall right (opposite of the blacked out crescent) of where his crosshairs are. It's an optical thing.

Except for one item: IF the next shooter(s) can adjust their position to eliminate the dark shadow altogether, the zero is STILL zero.

Yes, of course. You can always (well usually always) contort your body to the right position to have the zero. Problem being, that being contorted doesn't lend well to accuracy.