Very helpful and thought-provoking comments, helpful and useful!
Question: When you say that the Peguri or Atzarin might not be "fully chromatic" I'm not sure what you mean. Peguri has all of the notes contained in its range. Atzarin does miss one note near the bottom (I'm just talking right hand for simplicity) and one note near the top of the range, but within that it has everything.
I agree that the Peguri is definitely not fully isomorphic, since it has two different coexisting arrangements, and I GUESS you could say the Atzarin isn't fully isomorphic in that you have to learn three, for example, major scales. But that's still a lot better than having to learn twelve major scales like I've already done on piano, and the Atzarin doesn't change its mind in middle of things. You play the same patterns in all octaves. Same as a three row CBA in that regard - requiring the learning of three patterns for each type of scale.
As you suggested, I actually have printed out patterns in "life size", and tried them out, and I need to try that with Hayden/Wicki. It helps, but you're never sure what it will be like in real life. For instance, how hard will it be to reach a note given that your hand is restrained by a strap.
I want to be able to play any piece in any key, and the easier to learn the better! Isomorphism would probably help - I did have a CBA for awhile and found it workable, especially because I could use my thumb. But what I don't like about accordion is that it strongly favors the right hand, allowing it free movement, while the left hand is given the task of keeping the air flowing. I like the more equal approach that concertinas and bandoneons follow.
I have a borrowed bandoneon of the traditional Argentine persuasion and I'm working with it. It's crazy! However, the patterns, once learned with much effort, do seem to fall naturally under the fingers. Of course that probably just means I haven't yet tried the patterns that are difficult on it. I imagine every approach favors some patterns over others, but none that I've encountered are more loco than the traditional Argentine bandoneon.
Anyway, I'm glad I joined this group. I'm an accordion player who wants to play bandoneon but I love all free reed instruments, especially the very portable ones! I have a close to complete collection of "Concertina and Squeezebox" magazine.