Fanta Lemon
Fanta Lemon IS present in Italy, but not so common. We prefer other brand for lemon soft drinks: Fanta one is probably too sweet for Italians.
I learned in Italy that one CAN enter a ZTL (limited traffic zone) when active to enter a garage.
Remember that ZTL are local areas, so each city has different rules. In general yes: if you enter into a garage or to an hotel within the ZTL they tell the plate number to the police and you are not fined to enter and exit. In any case is better ask to your hotel/garage in advance to avoid fines, because in some cities you have a route to be followed or some areas where you cannot go even if you can access a general ZTL.
I saw signs about speed checked electronically somehow
90% of times are general waring, often to say that the speed could be electronically checked (maybe with mobile cameras installed on police cars). There are fixed speed traps somewhere, but not on 100% of the time. In general follow the locals behavior is a good advice. Or better respect the speed limits, but sometimes are really crazy!
I was never asked for my international driver’s license
IDP is mandatory if you are a driver from outside European Union. In the past was more important. Now we have a lot of driving licenses from everywhere in Europe written in English, so if your DL is written in that language almost nobody asks for the IDP.
But remember: if there is an incident or a formal complain and you don't have it is like driving without license! So you are fined and the car is taken by the police! Or maybe the Insurance Company use that matter do don't cover the damages!
on the main A1 Autostrada I never saw a speed limit sign
The speed limit is in a big sign when you enter the Autostrada. There are all the limits and the vehicles not allowed to enter. Being a big panel always present almost nobody pay attention to it! :-D
On the most of Autostrada (A1 for sure) there is the "tutor system". Every some Km (20 or 30) there is a camera who can be used as a standard speed trap, or sometimes are on several ones and they do a calculation of the average speed reading your plate number.
In general, as others say, if there is no sign the speed limit is the standard: 130Km/h on highway, 90Km/h on rural roads and 50Km/h on urban streets.