Before you modify 12 inserts, let me add another chapter to this story.
The insert that DJ modified for me by grinding a bit off it is the 1"x8tpi shown here on the left.
After grinding it was seating hard up against the G3 chuck and all run-out disappeared.
I believe this insert was manufactured by Vermec, it was supplied by Carbatec with the Nova G3 chuck, no packaging with the insert so I can't be sure. It does however look similar in design to the Vermec insert I purchased recently for my VM120 chuck.
A little while ago I purchased a new lathe, which has a M30x3.5mm spindle thread, so I had to purchase a new insert. On the way home from H&F with the new lathe I stopped at Carbatec and bought a M30x3.5 insert for my G3 chuck, which is the one shown on the right in the first picture and is made by Teknatool. Notice that it doesn't seat hard up against the shoulder of the chuck, yet the chuck has no discernible run-out.