I've finished my analysis and I've learned two very important things.
1. The Screen Balance app uses a technically inaccurate calibration-by-eye technique. When attempting to set a white balance by eye, I would make only small adjustments, indicating that white balance on my Pixel 2 XL was already close to being optimal. There is really no way to make any accurate color or white balance settings. Its just purely guessing at settings. For me the Screen Balance app has very little value. I've uninstalled it.
The real problem for me was that color accuracy was only OK on my Pixel 2 XL. It was really quite obvious that when comparing the same photos with my color calibrated PC screen, there was enough of a difference to not have any confidence in my Pixel 2 XL color accuracy.
2. The most important thing I learned was that my Pantone X-Rite i1 Colorimeter has an Android app that works with my Pixel 2 XL. I simply use the small USB C to USB adapter that comes with my Pixel for the connection. The Pantone Android app is called "ColorTRUE" and is available on the Google Play Store. I've now calibrated my Pixel 2 XL to "exactly" match my PC screens. The calibration is automated and takes about 6 minutes to complete.
There is now absolutely no difference to my eyes when comparing the same image on my color calibrated PC and on my color calibrated Pixel 2 XL. I couldn't be more pleased!
If anyone is interested in having true color accuracy on your Pixel 2 or Pixel 2 XL, on your PC, or on any other supported Android or IOS device, the price of the Pantone X-Rite i1 Display Pro Colorimeter on Amazon is $258.
Had it not been for you guys I would not have learned that my Pantone X-Rite i1 Display Pro colorimeter could also be used to accurately color calibrate my Pixel 2 XL. Thank you!