I agree. Without an overly complex and expensive design/construction model, melodica players will not be able to change the angle of attack of their breadth on the reeds. The reeds are too far away. So we must manipulate the sound after it is made.
I can think of numerous ways to keep the keys and the valves independent so the valves can reside in an air-tight chamber. Valve design can be very simple: think valve in an overhead-cam, internal combustion engine. You have a valve with stem, spring, and spring keeper.
Yes, I suppose you can manipulate the sound from each valve in the valve-last model. But there is nothing stopping one from created a cover than has chambers for each of the exposed reeds in a reed-last model either.
In any case, there are scores of possible designs. Daren will certainly have his hands full developing and testing prototypes that are simple, effective, economical to build, and which provide a high quality sound. The basic overall configuration that provides the most flexibility and expressiveness must be figured out first.