Yes, I can see the problems with the button process would be far worse than those of a thread mill or your sinebar machine or the geared process. I'd believe an order of magnitude.
Since I now earn my crusts and sour milk by engineering turnkey projects for the world's premier builder of CMMs, it would be fun to hear how the twist measurement is instrumented. (Not that I don't know how to record rotation as a function of displacement.) A slightly undersize (and rather short) button on a rod, passed thru the bore with an encoder on the rod, maybe?
Since I've got you on the hook, and you obviously have the professonal creds - is it current practice to let the lap actually extend thru the muzzle end as a convenience, knowing that the fixtures will be cut off? The conventional wisdom of ages being that the lap was to be held short of the muzzle, I've always followed that dictum. But then I'm always lapping an existing bore or a rebore, not making brandy new ones. I even go so far as to do my charging at the breech end. Which means that about 3 inches of the lap never gets charged directly.
Now, about this rifling machine - you are still USING it? Send me some pics! (Lots of people collect guns, not many of us collect machine tools, but in a small way I am one of those. Makes it awfully hard to relocate, let me tell you. ) I'd have thought even a rifling machine would be CNC by now . . . .