A melodeon is only "logical" if you start with a diatonic row or rows in the key you want to play in, and basses to match

You can't play a D major chord on *that* melodeon because it's got very weird basses!

And you can't play a right hand D chord on a B/C box either - but I'm sure a B/C box with the basses retuned for playing mostly in G and D does have a D bass.

A standard D/G with accidentals is good for D/G major, E dorian, A mixolydian and some tunes in C. I have learned a to play a tune in C minor on one but it was excruciatingly difficult and I couldn't do any proper chords or basses for half the tune.

Anahata