I undid the top mount for the shock and, as far as I can tell, it has enough rubber-mounted play in it to not be putting any significant sideways leverage on the linkage. So... I'm still at a loss to understand the cause of this scraping between the link parts. Maybe I'll remove them again and do more than just an eyeball test for straightness. The bike tracks straight hands-off so I doubt there's anything major twisted or mis-aligned.