Jim, I am glad I asked this question here. I know a lot more about the camera as a result.
Just before bed, I did a few tests last night. It appears there are multiple ways to adjust the green/magenta, including the option to save presets with the adjustments and even name the presets.
In my test, I created a d-1 preset with Magenta +6 and named it that way. I named d-4 as Default with no built in adjustments. And I was able to press the WB button and toggle to PRE, then pick from any of my four saved versions.
There was also an option to do an automatic WB capture/reading and save it as one of the presets.
And as you mentioned, there is the other option to set the WB in both color groups, including the option to adjust the Blue/Yellow in much smaller increments.
We are definitely on the same page with different ways to getting to the end.
Later today, I will look into the other WB options.
In some of the night sky, milky way photos, you often get a green shift in the sky, especially along the horizon. Being able to set the magenta higher would offset the green and make the sky richer.
I never knew there was a White Balance "bracketing" option either.