Fantastic, Fred, both in terms of write-up and in terms of timing. I've actually been messing with a toneboard a friend asked me to assess the last couple days, and will apply some of your diagnostics.
This particular one has a flat tone channel (no slope), which I hypothesize is causing tuning problems regardless of what reed variation (thickness, flexibility, width, length) I throw at it. All of them have to be tumed too low/deep (too much curvature) for my taste to get a good, easy quack and chuckle, yet still run too high and squeal when pushed with much pressure. I was going to use some clay this weekend to simulate a slope to prove or disprove my suspicion, but reading this recognize you've probably been down this path already. I'm going to test anyway, but would a flat tone channel display these symptoms? If not, any other ideas?