VEEERY interesting. The fact that the rails were noy damaged is a very important clue. The chain would not have been stretched the first time. Based on the new info plus the fact that the offset keys are shearing off points me in another direction. In the scenario I played for you, the keys are never sheered off. I suspect the cam is binding. If the head was overheated and warped on the top side, it could cause the problem. You could also have a bent cam. Seen it on a 560SEL. A professional mechanic constantly checkes fit and ease of movement during assembly jobs. With the head torqued and the cam bolted down, you should be able to rotate the cam easily with your hand. No pliers should be needed.
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