I'm using a Skywatcher f5 coma corrector that doesn't screw particularly well into the Orion TOAG I'm using - setup is Coma Corrector, TOAG, 16.5mm spacer, 20mm filter wheel and then ASI 1600MM-Pro (6.5mm). I think the extra room between the TOAG and Coma Corrector might be causing an issue (need to measure this but assumed it was around 12mm). However the focuser has been replaced on the Newt and the primary has been moved up a little. My thinking is to make sure the focuser is square to the tube (necessary?? and if so how to do this??), and make sure the secondary is centred under the focuser tube, then collimate. Then also accurately measure the distance from imaging chip to Coma Corrector to aim around 55mm. I gather form what you're saying Glenn I could, for sake of alignment, get rid of the coma corrector and use the coma to help align the optical and mechanical axis?
Here is the Astrobin link so you can see in detail how the background stars distort -
https://www.astrobin.com/345100/