Hi Buttonbox and others. Very interesting discussion and I would like to make a few points if I may.
MELODEON = Treble notes are different on the pull and push ie. Push G, Pull A.
ACCORDION is the same note, push or pull.
Therefore an Irish B/C/(C#) is actually a chromatic melodeon and a D/G is a diatonic melodeon. However, I don't think anyone will ever convince the Irish to change nomenclature.
Continental Chromatics have the same note in or out and are therefore accordions. This is regardless of number of basses. Piano accordions obviously have the same note in or out.
The Jimmy Shand box was a Hohner Gaelic Ø B/C/C# Chromatic melodeon with a 72 Stradella bass and I have one for sale.
I use a 2 1/2 row Castagnari Mori and use the 1/2 row for accidentals used in some tunes (Jump to the Sun, Horses Branle etc) and have some notes the same as the G and D rows but with the push or pull reversed. This means I can play these notes without having to stop playing the bass when playing a run and make the tune flow better.