I tried to keep myself to the written suggestion of Hugo Aranguiz, which came with the bandoneon, namely that before attempting to play any piece I should learn the scales and other basics well; the lessons in Madrigal's Método also suggest such progression. As far as I recall Ben also advised a similar path, and Maxi also wrote me he decided to only practice scales for his entire first year before turning to any actual music. (This reminds me too well to the legends about leaders practicing tango for years amongst each other before showing up in a milonga for the first time. Damn milongueros.) A common element of all of these advices were: patience.
I'm sure this is good advice. The problem, I think, was that since I don't have any serious background in playing a musical instrument I didn't have any frame of reference to measure my progress, and I didn't quite see how the steps I take fit into a path which eventually leads to bandoneon heaven. It's easier to be patient when you know better what are you waiting for. At any rate, I need to admit playing scales became a bit too monotonous. I started to spend way too much time with simply taking pleasure in the loud voices I can get out from the instrument; eventually I comforted myself with the idea that when I get back to Hungary I'll have the chance to contact other bandoneonists and restart from scratch.
Now I'm back. Getting through customs would deserve it's own tale (at a certain point I needed to play Boci-boci tarka to convince the customs officer that I'm not bringing the instrument for sale - apparently I would have needed to own it for at least 6 months to avoid harassment). I was lucky, a few days after I arrived Divertango had another great performance, and so I took my bandoneon to meet Tamás Radnai. He is both knowledgable and enthusiastic about the bandoneon and got quite excited to see my instrument. I had a couple of things to take care of, but we agreed that I'd take lessons from him.
Divertango playing Piazzolla's Libertango.
And so a week ago I got started again. Tamás is an experienced teacher and the classes are really useful; I basically get an ideal blend of musical theory with application to the bandoneon. Theory-wise I've (re-)learned to read scores, and it's nice to finally have a theoretical understanding of musical keys, of the minor- and the major- scales, of the composition and function of accords, etc.; luckily he is very patient with the sometimes maybe too annoying questions of a physics-minded guy. Practice-wise we started doing small finger-exercises for basic tunes such as changing between playing quarter notes and half notes, and we focused on opening the bandoneon uniformly to not to have changes in the way different notes sound. Apparently I have a tendency to rest my left arm on my left leg, a bad habit probably due to that it's easier to hold the weight of the bandoneon that way. Also, we did some basic exercises with changing between legato, tenuto and staccato; i.e. playing the same three-note tune with both hands but one in legato and the other in staccato.
Two online resources I recently found extremely useful:
Practicing note-names online: http://www.emusictheory.com/practice.html . After spending a couple of hours with "drilling" I can now recognize sheet notes in 4 seconds on average. Not enough to read continuously of course, but better than not being able to read at all. As an exercise I identified the notes of Bahia Blanca.
Bandoneon simulator for Mac and Windows: http://zztt.org/bandoneon . This is what I'm planning to use now to learn the chromatic scale. Couple of notes don't sound the same pressed on the virtual piano and on the bandoneon, and the bandoneon buttons don't have the row/# numbering on them, but otherwise it's really great!
Finally a practical advice: get a blanket which you can put between the bandoneon and your supporting leg, it prevents the bandoneon from ruining your jeans!