Thanks. I appreciate the assistance. Here is what I'm taking away so far
1. With a color camera the appropriate x-scale and y-scale factors to enter are twice the individual pixel dimensions. In my case I should enter 0.52 rather than 0.26 for each X and Y scale. Is that correct? (Not sure I understand why it would be.)
2. When you later attempt to redo a platesolve on a saved image, and when it reports its estimated actual image scale, it is reporting the binned scale. In other words, the reason my reported scale was 1.04 is that the actual unbinned image scale was 0.52 and I was using 2x2 binning. Is that correct? (If so, it might be helpful to modify that window to emphasize that the reported scale is binned scale.)
3. I may just be asking too much to get good solutions using an f/15 telescope and a camera with small pixels, given my Bortle 5 skies and with the moon out. One option is to use a camera with a larger pixel size. As evidence of this, in using a similar camera on my 130mm GTX I still had to do 2x2 binning to get easy solutions. I use a Lodestar X2 as a guide camera on my other setup. It has 8.2x8.4 pixels. I can try that instead of the ASI1600MC-Cool.
4. Other things to try: more tolerance around image scale, less sigma above mean, longer exposures.