Italy in July/August 2017

If you're not seeing Florence, rent your car in La Spezia; Roberto has often posted details of this (such as getting from the train station to the car rental offices).

If I read this correctly, you're planning to take an overnight train to Rome, arriving in Rome the same morning as your flight home. Unless you feel very lucky, don't do this. If there's any disruption, you'll miss your plane. That's not to mention that most don't like this train (to be fair, a few do). Instead, I'd fly from Catania Rome the day before; in fact, that's what I did.

To find all flight options, look at Skyscanner: https://www.skyscanner.com/

Note that Alitalia has a more generous carry-on allowance and you get one free checked bag, so it can be cheaper than a "budget carrier" on this route. Also, for this route, it's cheaper to book on Alitalia's "international" website, which has an English option: https://www.alitalia.com/en_en/ This gives you the same prices as their Italian site, which is only in Italian. Their US site gives MUCH higher prices - for the exact same flights.