Using TIM as an example:
1. Yes. You buy a SIMcard for 10 euros and it comes with the TIM12 plan preinstalled.
2. Indeed - you can text the option, but usually in the shop they will add it for you. You really need to go to a shop. There is an imprssive amount of paperwork that they perform. It took about 15 minutes in my case. Make sure you have a codice fiscale. No codice, no simcard.
Make sure that for the plan you want there's enough credit on the simcard. The included 10 euro won't bring you very far in terms of mobile data. If the credit is enough, you can activate a plan, through the SMS codes or one of the other options.
3. I have no idea. There's a mobile site where you can log in with your new mobile number and review and manage your options.
4. Yes. I added it when I bought the sim. If you hve enough credit, you can add it with sms messages, but it is an expensive process - the messages you send are charged, and you'll send a few, e.g. at least one to change the language in English
5. It is for prepaid indeed. They renew the option every month and use the credit that is on the card -reload on time. If there's not enough credit, the option will not be renewed and you fal lback to the standard prices. Once there's enough credit, it will autoamtically renew itself unless you asked to stop it through one of the many options.
In general, all over Europe, when you buy a regular SIM, you can call and text anywhere. Only the price is different according to the plan you use.
You can surf the TIM website in more or less understandeable English if you use chrome and the translation option.
The best you can do is walk into a TIM or Vodafone (or WIND) shop and let them help you. You'll need a salesperson for the sim anyway. Mostly they can help you in English.