go back to simple analysis ... if your rotate the flywheel backwards and cam gear isn't moving at all like in your video, its got a sprocket issue. Shouldn't be any way the chain has as much stretch in it to take up the amount you turned the flywheel. whether the drive sprocket is split or worn in one direction on the inside bore, mostly stripped of teeth, or the timing gear is slipping on the cam (dk if that's possibly) there's something wrong at that level. Not saying the chain isn't stretched as well, but that's a lot of rotation!
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