So if they're doctoring the CD mixes, then that's one contributory variable verified. But, does that automatically negate the resolution as another contributing factor?

No, it doesn't but until the hi res is the onl;y difference no one can know. So far, no such demo has been offered by the people in the know and capability to do such a demo. One has to wonder why that is. Perhaps they know the answer just as the wire companies know the answer, or the writing on the wall.

. Like I said, I've yet to encounter a high res disc that does not sound at least as good as the CD version.

Well, I surely hope that they do sound at least as good as a CD.

The reasons why that is so do not matter, so long as it is so.

As long as only preferences are offered for the hi res. Otherwise, if testable claims are made, then evidence is in order.