1. I noticed that the high B of the first keyboard does not sound, with no stops engaged. Most likely a console problem, right?
See above. It is usually not the console; the problem is in the windchest.
2. Between the two keyboards, there are small pistons with certain dynamics indicated (p, mf, f, etc.; see attached photo). Are they an alternative to the pedal (which operates the light indicator typical of Mascioni organs, see photo), or are they independent? If they are independent, how do they affect the sound?
Those are called fixed combinations. They are indeed combinations of stops, defined by the manufacturer, to activate stops based on the desired sound expression (pianissimo, forte, fortissimo). They are a "ready-made meal" suitable for a quick recall of a sound and are very useful for those who do not know how to register. I never use them.
I would draw attention to the two regulating pedals located above the pedalboard, in the center of the console, which are called expression pedals (staffe).
The first one on the left (always, in every instrument) is the GRADUATOR (expression pedal). In this instrument, you can see its action specifically on the indicator you mentioned. This "gradually" inserts all the stops, from the foundation stops to the mutations (practically, if you look at the row of stop tablets, it inserts them from left to right one by one). This is also usually set by the manufacturer.
3. There are two small pistons for the "Automatic pedal" function, one for engagement and the other for disengagement. Despite my attempts, I haven't been able to understand quite what they are. The same goes for the two "Pedal" pistons, one on the first keyboard and the other on the second.
The automatic pedal does nothing more than make the pedalboard play the lowest note you press on the manual. For example, if you play a C major chord, the console will automatically make the pedal play the note C. Obviously, you must engage the stops on the pedalboard, otherwise no sound will come out. This is also useful for those who do not know how to play the pedalboard, but it is only useful if you want to reinforce the fundamental of a chord in the bass.
The pedal pistons should serve (then check, because I am not sure about these Mascioni consoles) to link that manual to the pedalboard. In practice, you play the stops of that manual on the pedals.
4 and last. What are the cancellers (annullatori)? O.o
The cancellers, as the name suggests, cancel the function of a family of stops. They do not allow them to be activated.
Let me explain with a practical example: we said that the graduator adds stops in sequence. Suppose that at a certain point the graduator of your instrument inserts the reeds, but you do not want them to be inserted as you increase power. You cancel them! That way they cannot be inserted.
The same applies to other cancellers. By cancelling the family of stops, it is as if you had physically removed them from the instrument.