Fretboard widths and wood chocies
Instruments- discuss specific features, luthiers, instrument problems & questions
Definitely agree with Ken and Andy. Fret board width and VSL is not an absolute. What one musician finds ideal, another may find unplayable. Play many different instruments and find what works for you, or, if you build to sell, make a few different models and give musicians a choice.
As far as wood, same advice. Find the model of dulcimer you enjoy building, then find the wood that you believe sounds best,..., and ignore the people who think otherwise. Music is in the ear of the hearer, build what sounds good to you. Best example, cherry. Some say it produces a sweet sound and others say it over emphasizes the treble. Does it sound right to you? Then it is.

To Rod... and our other Absent Friends.
It sounds Celtic to me, too, but Ken may be right that there are pieces of a couple of tunes there. Part of it is a melody I know, and I may pop in in the middle of the night when I remember it's name. But I don't recognize that descending ending/turnaround.