the lowest notes of a chord are loudest, but I have noticed that this can be (again, psycho-acoustically) corrected by keeping the lowest notes shorter
Yes, or can be played as ‘grace notes’ – very short notes played just before the main note, which give a sense of the harmony. I enjoy working with some of these melodica characteristics. I see it as an instrument which really excels at monophonic playing. When composing or arranging, I like to find ways of implying the harmony through the melody. I find this approach works really well with Irish traditional music and solo Bach, both of which rely on a monophonic instrument providing the whole hormonic experience, so to speak.