Insightful comments you have there. I have several insights to add but I am in Portland, Maine, for the weekend without a computer so I cannot write easily. I will add some thoughs when I get back.
But I will say that because of its small size, the closeness of the reeds to the players tongue and mouth, and the ability to modulate the sound acoustically with infinitely variable hand and finger movements, the harmonica is the most expressive free reed instrument.
Daren, these principles deserve some thought as you brainstorm melodica design.