Wow! Thanks for all that, Richard. Lots to chew on there.
If I had to guess, though, and that's all I have, I think it strains credulity that that much scientific thought went into the design of these strings. The late John Pearse was a musician, not a scientist. Ditto for his ex-wife, who is still involved with the company and was a professional dulcimer player. So she surely had input into that set. Perhaps it's something she experimented with, or perhaps she stumbled on it by accident, or perhaps it's something another dulcimer player suggested that she try. In any case, however it happened, she probably liked how it sounded and that was that. All of that is sheer speculation of course, but being a musician, I know how I think about such things. Bottom line, how does it sound? If it sounds good, it is good. I don't dwell on the science. But in this case I was curious, and your explanation may well be the correct one. I just find it hard to believe that much forethought was put into the design.
I do think it is interesting that you use a mismatched bass course. This is the first I've heard of a maker using a mismatched course in a purposeful way.
Maybe the reason that John Pearse did not answer my query is that, the truth be told, they just don't know the answer? Maybe John said that's what he wanted, and that was that?
I don't want to change strings on any instruments I have right now, they aren't needed. But I am picking up a recent used purchase soon, and I usually put on fresh strings as a standard procedure. So I will use a set of these and report back my impressions.