But the runners- or, at least one of them- didn't check up.

If R2 is close enough to run into the shortstop, then he's close enough to get himself tagged out by the shortstop.

If there was a "mild collision" between a runner and a fielder that had posession of the ball, if I were the defensive coach/manager I would contend that a tag took place and the interference on the throw was perpetrated by a retired runner. Guys in blue may not buy it but I'd have to at least try.