Thank you to all for your help.
I managed to set up the front brake too (with a longer cable and a reduced yoke angle).
These are the photos:
Front brake with joke lowered at the maximum possible position
Rear brake
I still can reduce a bit (1 or 2 mm) the distance between front pads and rim, however now the bike brakes better because I don't have to squeeze the handles with all my force to get a poor deceleration. Now the effort on the handles is less and I get a better deceleration.
I asked to the local shops for the Kool Stop pads, but they don't have them. They proposed me the Shimano BR-CT91 instead. Are they good too? What about pad length? Is it better to have longer pads?
The bike now is already usable, but better pads could give a better deceleration and if they have a longer pin, I could try to increase the cantilever angle again to get more mechanical advantage.
I noted that cantilever arms are free to move about 2 or 3 mm along the attachment. If it is not normal I could add a washer to limit or block that movement.
Moreover the stranding cables are still the original ones. Could changing them with newer ones improve the braking power again (the seller at the local shop told me that they are going to arrive tomorrow)?