Yes. Invalid only means at that point in the pipe, the found illuminant cannot be approximated by daylight or black body. Remember a couple of color massaging steps happen before in the pipe.
Reflective surface under spot vs. even light.
The reference values for grey patches are not R = G = B either. Grey is never completely achromatic on the targets.
The adaptation used to be changed depending on what illuminant was found, but I changed that just before release.
Yes. Gamut compression preserves luminance and sort-of preserves hue, so adapt it to the current gamut in your picture.
One thing you could try is to calibrate your white balance coeffs (in white balance module) to D65 by shooting a white patch displayed on your computer screen (if it is calibrated). On that test shot, use the spot WB picker in the central region and save the coeffs in a WB preset. Then calibrate from color calibration with the color checker with these WB values set in white balance module. I have found that this decreases the delta E a bit more, and the illuminant readings should be closer to the expected ones.