I tried taking a closer look at the 5C chuck today, trying a feeler gauge between the chuck itself and the backplate. Smallest feeler gauge I had was 0.0015" and that would not pass through the two faces. So it doesn't appear to be something simple like grit between the backplate and the chuck. Just for giggles I took a look at the face junction between the spindle and the backplate of the chuck, and there I saw daylight. I could fit a 0.004" feeler gauge easily, but the 0.005" was a NO-GO. I tried retightening the camlocks, and even removed the chuck and removed the camlock lugs on the backplate and raised and lowered them one turn, which just made the camlocks themselves not really lock up.
I took a look at the video I did again, and I was seeing just a hair under 0.001" at the edge of the backplate. I dunno, would 0.001" runout at the backplate equate to 0.0065" at the nose of the chuck? Looking again at the video, the runout gets progressively worse as I check along the length of the chuck. Is this the problem? The chuck backplate is just not registering against the surface of the spindle face causing the excessive runout?