Ok here is the result of my local testing version to deal with negative luminance:
E Gamut L-C demo blue Guard Rail:
E Gamut L-C demo blue AgX with Guard Rail:
E Gamut Sweep with AgX:
And the Blue Car Light:
What do you think of these results? The blue car light still seems a bit darker than preference for some reason, but all other tests are giving positive results, so might leave it be?