Hi Windstrel,
Welcome to the forum.
Manu Maugain's course runs to three volumes, as you may be aware, and he uses a more modern approach to fingering with the right hand. The accompanying CDs are quite handy, but with your previous musical skills you may not require them. There is a lot of myth and mystique about CBA, but if you lived in France or Portugal they are more common than PA boxes.
I've never even seen a Roland accordion, let alone played one. You're probably as well to keep it programmed for C system, to comply with Maugain's teaching method, although later on you might want to try B, or any other systems it offers.
My advice would be to stick with Maugain's method and move onto his volume 2 when ready. If you don't do that then you'll probably set yourselves back some. I know as I had to try and teach myself the whole show with very little knowledge of musical theory, and kept reading that somebody else's method was better. Each new method meant virtually starting from scratch, and I think I maybe tried about 5 different ones before I settled on the Ferrero method, which is also French. I wasn't able to work through the whole of his course as I couldn't understand half of the text, and there was no such thing as "gobbledegook translate" or the internet in those days.
Very recently I became aware of the existence of a fairly local CBA teacher, who has only been playing CBA for 8 years, but had played PA for 40 years before that. I decided to go back to school and discovered I had developed every known accordion technique "fault" there is. I had even developed some faults of my own invention, that he has never seen before. However, one thing is very clear, and that is there is absolutely no "correct" way to finger a CBA. He shows me ways to use all 5 rows to minimise hand and finger movement to improve my accuracy, and I show him alternative fingerings concentrating on the outside three rows, so that my accuracy isn't compromised by straying into the uncharted territory of something called a "5th row"! He hates to see me with my thumb on the side of the treble keyboard, but that was the way I learned it. I'm slowly getting used to resting it on the next button I'm about to play, but I'm 65 and have been playing (as an amateur) for over 30 years. It's obviously his way or no way, so my 5th row buttons are now getting a hammering. (I have two boxes with only 4 rows). Incidentally he doesn't use any teaching method whatsoever, but devises fingering appropriate for every tune and that's what we both use. We'll tackle just about anything but the most difficult French musette tunes, and I'm happy enough with that. I certainly couldn't have gone back to "page one, lesson one."
One thing I will say is, if you start as late as I did (about 32), you have to appreciate that the finger and hand positions some of the virtuosi use will simply not be attainable for you. The aging process starts to kick in relatively early, and those tendons and other muscles have already began to lose their flexibility. Despite that you should get a lot of fun out of playing, especially when you get to find out those little tricks you can put into a CBA that are almost impossible for most PA players.
Good luck with your progress, and if you can find a local CBA teacher, it may be worth getting acquainted. It could save you a lot of "re-learning" later on. A lot of people, myself included, thought the accordion looked easy to play. I thought it was just a case of adapting my guitar skills to another instrument. For about the 143,887th time in my life I got that one distinctly wrong!