If we roll out of Florence on March 30th around 10:30am, we can be in Monterosso(CT) by 1pm and then stay the night and take a late afternoon/evening train to Rome on the 31st. We'd then be in Rome, April 1st - April 6th(5 full days), flying back out to Paris on April 6th(one day before our departure out of Paris). I think this could work? :-)
If you have 4 nights in Rome (3 full days), you can add a night to Monterosso, if seeing the coast is important to you. Obviously it depends on your personal preferences, but with 11 days in Italy, spending 5 in Rome, I would consider it to be a "Rome heavy trip" in my book. If I had to allocate time on a 10-11 night trip, I would not give Rome more than 4 nights, otherwise I'd be shortchanging everything else.
wondering about driving in the Florence area or even from Florence to Siena and back? I've backpacked for 3 months or more in Europe in the past years and have never ever driven and wonder what the thoughts are on that? Difficult? Not worth it? It'd be nice for flexibility access to Siena or just around the Tuscany countryside from Florence.
If your intention is to visit just Siena, then a car is totally unnecessary. The bus will take you there in 75 min. A car might get you there in less than one hour, but then you have to add the time necessary for the rental pickup/paper work, the time to find your way out of Florence, the time to find your way to Siena city center and find parking. So in the end, when everything is said and done, the car isn't any faster. Don't forget that Siena also has a ZTL (Limited Traffic Zone) in the historical city center therefore you can't drive inside and must park outside the city center (only residents with permit can go inside the city center, in some parts of it). I think to visit Siena only (which will take most of your day), you are better off by using a bus.
A car could be a consideration if you intend to visit the Tuscan countryside or small villages, e.g. the Chianti hills, San Gimignano, Monteriggioni, etc. In that case, a car makes sense because buses to small towns are infrequent. Driving in Tuscany is not difficult. There is a freeway that connects Florence to Siena, but to drive to the small hill towns you'd be driving primarily on narrower two lane highways, which tend to be very curvy because Tuscany is not a flat place (more than 80% mountains and hills). If you are comfortable with that type of driving, then you won't have any problems. I don't know where you live, but here in the Bay Area we have similar roads (or worse) both in the Sonoma wine country and in the Santa Cruz mountains in the SF peninsula. Most small size economy and compact vehicles in Europe have manual transmission, so if you only know how to drive automatic, you might need to upgrade to intermediate size, where they have both.