1) No problem... if you find the shop!
For example I fear there's only a Vodafone shop at FCO airport, maybe TIM, hardly Wind or 3... (but I could be wrong)
2) Yeah, first minute you pay set-up plus full minute (or usually 30 second). Of course with 1/1 tariffs you pay by the second.
3) Ask the customer to activate it (BTW it's "Call YOUR country", not my :P), it's free and there's no problem with any plan (I suggest you Wind12 or SenzaScatto). Consider that it could take up to 24 hours before the option is active. Also tell the customer to set you the English menu when calling 4242. Wind should have the best coverage in Tuscany while should have no big problems in the other places you are visiting.
4) Uhm... I never use it, but usually with Wind you just have to dial 4040 and follow the instructions, which I hope are in English too... but I used it last time really much time ago. Calling 4040 has a fee, while receiving messages is of course free for you.
5) Buy it in shops here, it's much cheaper and much better for registration problems. With Wind you have to choose which plan you want (there are just 2, plus Wind5 which has a different starting kit) at the moment of the activation, the same should be with Vodafone, while with TIM you should have a preactivated one. Of course you can change, but you pay (7 euros with Wind).
I hope it's all, since I'm leaving in 1 hour!
Anyway if you need other help there are other good Italians here!
Enjoy your trip!