The results are in.
Everyone can breath a sigh of relief. All is good.
My best tester was available and I changed the test. Instead of asking if he could see the header, I told him that I'd hidden the name of a creature in the file. If he could see the name and tell me what it is, we were good. He responded correctly: Banana Spider. So, he could see the header.
I then asked him if he could set his screen reader to transcript mode and email me the transcript. This is a feature intended for sighted persons to use. I don't beleive it was an easy thing for him to do, but he did it. I was able to go over the transcript, and the CHM format works just fine without a skin. No problems seeing or identifying the headers, graphics, links, buttons, or anything else.
I then sent him the file with the skin applied. He sent me another transcript. The only things missing are the page navigation buttons (such as top and next) that would appear to the right of the page header. I'm not going to fret about that.
What is key is that my most reliable tester was able to demonstrate that the CHM format is compliant with the United States Access Board's 508 Refresh for 2018 when no skin is applied. And, if someone insists on using a skin, it is reasonably accomodating for an outdated format. The page navigation buttons are not required, just more convenient than the navigation tree.
Those of you from England, be proud. My best tester is one of you.
Tonea