Yes, I can't disagree with you. For the sake of my own sanity, I've reset my iPhone 6 Plus and shipped it off to my sister now so she can use it instead of her iPhone 4S. So I can't compare anymore.
I don't have a luminance meter, but I have no doubt you are correct. I've been spending the last three months doing all those comparisons you mention. And I see the same things you do.
Apple's engineering team's explanations have from the beginning seemed like "The Emperor's New Clothes." Like we are made to feel dumb if we don't see what isn't there. I think the advertising is incorrect, and their explanations make no sense.
Like I said, I was given a refund and thus had the option to go elsewhere. But I decided, for me, to just drop the issue for now and live with the color tinting change (select the last option, turn intensity all the way down, turn the hue slider to about 70%) which is liveable for me at the brightness levels I normally use.
But I agree with you. I just couldn't obsess over it anymore and didn't see an easy alternative, though I went to look at Sony and Samsung models.
I have an upcoming trip to the U.S. and the fact this is now an unlocked model helps. Basically I compromised. And Apple did give me a refund, so they didn't leave me in the lurch.
But the advertising is not correct. I hope Apple owns up to the problem and offers a true fix at some point.