Any chance you can slip something up the side of the shaft into the adapter to jamb it into place, to hold it while you screw it off? Something to shim it - thin screwdriver, feeler gauge, thin strip of an aluminum can, etc.
I may try grabbing a can of keyboard duster, holding it upside down, and blasting it into the little bit of a gap to try and shrink the adapter a touch. After I replace it. May be able to save the knob and work on a better adapter with Jarrett @ Raceseng to save others this hassle.
I know you'd like to save the investment. But, they really should replace your knob knowing they've got some sort'a defect in design. I guess you could always use a small cutoff wheel on your air tool to cut it open just far enough to wack it with a hammer and chisel. It'd probably open up like a ripened peach.
