Three days later -
In the tank, rescaped with Fluorite, pH = 6.5, KH = 1.
In the sample container, with a small quantity of Floramax midnight black that contained 2 small white chips, pH = 7.8, KH = 4.
I contacted Caribsea and was advised that their 100% satisfaction guarantee would cover this, provided that I could produce a proof of purchase. I, not being a pack rat of pathologic proportions, don't have that. I wasn't seeking compensation anyway, just thought it was an amusing side note. In response to my comments about the product not performing as advertised, the excuse given was that it is a natural product. I'm not sure why they feel that producing a natural product (whatever that's supposed to mean) gives them the right to make false claims on their container, but there ya go.
In short, I had persistent problems with pH and KH rising, regardless of my attempts to stop them. After removing nearly everything from the tank (driftwood was the only non-substrate/livestock item left) I finally noticed white chips in the substrate. By chance there were none on the surface in the front half of the tank, though they were there when I dug for them a bit. I tested the chips, the tests indicated that they were carbonate minerals of some sort, and I rescaped with Fluorite. I threw out the old stuff except for a small sample which, after testing, showed the same behavior that I had been fighting in my tank for 2 months or so. I have not had the same problem in the tank since.