I had to look up “cassotto,” being relatively ignorant about accordions. Now I get it.
My observations are about the soprano Mylodica, and I have no idea what’s inside the alto.
I’m puzzled about the box warping, since there’s nowhere (in the soprano, at least) for moisture to escape from the inner melodica other than back out through the mouthpiece opening, which I guess means it should be stored mouthpiece end down. But if you’re blowing without a mouthpiece, maybe some of the air and moisture is entering the outer chamber, as happens with my Suzuki M-32.