It's a beautiful instrument - and I like the simple but functional layout.
I'm also interested in the sound quality of dulcimers, and I wonder about the contribution of the upper bout sound holes on this instrument design. The top plate where the long holes are cut would not be vibrating very much, and air resonance interaction with the wood would be less than lower down. It's hard to say whether there would be a second Helmholtz (rum jug) resonance given the shape of the instrument, and it's sometimes hard to excite them anyway in long holes (by blowing across them with a straw or similar). So it could be that the upper holes don't add much to the sound, and could be omitted without acoustic penalty if you chose. On the other hand, two sets of holes might allow both odd and even harmonics in the cavity standing waves as opposed to only odd harmonics with a single hole. That might make for subtle changes in overall tone. This is all complete speculation, but is easily tested. You could cover the upper holes with a couple of layers of masking tape, then remove the tape whilst playing. If you can hear a difference, then the holes clearly add something to the sound. If you can't really hear a change, particularly if you listen without looking, while someone else removes the tape, then the holes are really just decorative. If you judge the sound to be better when the holes are covered, then there is a definite reason to omit them.
The trouble with this is that better/worse is almost impossible to decide - even deciding whether there is a sound change is often not easy, and if you are looking at the instrument while covering/uncovering the holes, bias comes into play. But you could be lucky and it might be clear cut.