This latest test of correcting the collimation of the AT8RC demonstrates that it will produce a virtually coma-free image as seen in the image of M81 (Bode's galaxy) below. This is good! However, I screwed up on creating the new flat. My white sheet apparently got loose and I left the camera on high gain (1600 ISO) rather than 100 ISO where it was supposed to be. Doh. Before post-processing, the imaged flat was not what I was expecting. Ran the image workflow anyway just to see. Need to remake the flat.
I suspect though it may not help much as that even with a marginally imaged flat, the previous addition of the field flattener had significant impact on post-processing out the uneven signal levels due to optical curvature.
So, before reverting back to using the flattener lens, I will take another round using a deep sky filter for comparison.