Hi Roger,
I think you have done a pretty good job with the equipment and exposures at hand. I don't know what your local seeing conditions are but this often can dictate the achievable SNR in the small galactic features that you are trying to capture here. Galaxies are small and faint and can be that much more difficult to get a nice looking result out of.
Perhaps I can offer you a comparison image, similar object, scope (C11 f/10), exposure times, same camera (ST8-XME) in 2-3" seeing. Attached is NGC 6632 in Hercules, 12.9 mag, 3' x 1.1' diameter galaxy, that I did back in Ottawa in 2007. The full details and a linearly stretched 200% crop are on my website here:
http://www.faintgalaxy.com/ngc6632.htm
I used ~5 hrs of lum and 4 hrs total of RGB, but binned 2x2. The attached image is only cropped due to frame overlap from dithering. You can see the ST8 starts to pick up the field curvature and coma inherent in the SCT. The star bloat you mention is pretty normal for average seeing at long focal length (should it really be labeled star bloat if it is normal!?!). The NABG is not a problem in any way for this kind of image with no bright foreground stars. The ST8 loves more blue data since it's red response is so strong (>85% QE), and of course blue is always more affected by the seeing, always try to get your blue frames high in the sky and more of them than red!
The funny colors on different sides of the stars I think are due to small collimation shifts and varying atmospheric dispersion from capturing at different altitudes and orientations. I have them to a lesser extent in most of my SCT images also, it is an endemic problem at long focal length.
I'm also attaching the processing log for this one, lots of retries at the processing after going down blind alleys. I threw out a lot of frames here (14 L and 50 RGB) for cosmetic and seeing conditions.
One thing you could try is an Lab combine, I have always found it works a lot better than straight RGB combine, especially when the color frames are noisy but the L channel is pretty good, which I think is your case here. Note I have less color data but it is binned 2x2, which is big boon for f/10 + galaxy + ST8 + avg seeing. The 15e- read noise with color filters and long focal ratio needs very long subs to get sky limited. Binning 2x2 in avg skies really has no downside and gets you sky limited in a reasonable sub length in this case.
Best,
EB