I did get to play with my friend's Castignari, you're right, it has a very good action. I did not see the Beltuna, but have seen pics of them. The ones I've seen pics of had end plate engraving. I haven't found a pic of the inside. I'm definitely a fan of 2 up 2 down reed blocks. I guess the big reason most do the all stand up is to make them removable?
The 13 button, like almost all my boxes, was a custom request from a friend. It came at a good time because I was wanting try it. I was not happy with the action. To make the exposed rocker keyboard using wood, you have to leave just a little slop to prevent wood swelling to seize the rocker, and with the keyboard on that one, since the buttons didn't line up straight with the buttons, there had to be varying degrees of bend in the rocker arm. The varying degrees of bend created varying degrees of torque. It wasn't bad, but I wasn't happy with it. That's why I think most use the metal mechanical assemblies, to avoid the play needed in wood and give a smooth action even with a little side torque, but I'm speculating.
I really like the Harmonikas Dix reeds. I've installed 2 sets, and getting ready to use 2 more. They really did capture the sound of the old German reeds like those used in the Sterlings and Monarchs. But the reeds, being copies of cheap mass produced reeds, are not of the same response quality as the hand made Italian reeds. A trade off for the sound. BUT, they are a big improvement in response and volume over the old German reeds. I tell people to think of them as improved versions of the old reeds, not competitors of new handmades. I've also heard good things about their handmades, and am processing an order for that and some bellows now. If so, it will knock a couple hundred off the price of an accordion with the price difference between them and the Italians.
I may be willing to try a 2 row in the future, but not quite yet.