Rome 3 nights doesn't give you enough time in Rome. It's 2.5 days when you're jetlagged and after a long-haul red-eye flight. Rome is big, bustling and confusing. Give it another 2 days. If you haven't found enough to occupy you, take a day trip to Tivoli.
Sorrento I found Sorrento to be a good base, train to Pompeii, Naples, and buses to Positano and Ravello. It will take several hours to get there by train. If it's in your budget, consider hiring a driver and car for a day from Rome to Sorrento. Stop for a couple of hours to see the archaeology museum in Naples, then Pompeii and finally your hotel in Sorrento. It will take most of a day to get from Sorrento to Tuscany/Umbria. I would not spend less than 4 nights here, probably 5. Paestum is not far by car but it's a hassle to get to otherwise, especially at that time of year when there are fewer hours of daylight. I think a car would be more trouble than convenience in this area. For instance, Sorrento has a system of one-way traffic that loops around the town.
Umbria (Orvieto/Assisi) I visited in May and loved both towns. I wonder though how enjoyable they'll be in November with the real possibility of rain and cloudy skies. The historic centers of both these towns are up on the hilltops while the train stations (and probably car parking) are down below. That adds a lot to your travel time. If you do go, spend a night in each and allow 1/2 day to get to Assisi and 1/2 day to get from Assisi to your next stop.
Tuscany What do you want to do here? If you really love Renaissance art and architecture, 3 full days in Florence is barely enough. If not, you can see the highlights in a day trip. I'd base in either Florence or Siena and day-trip to the other. You can day-trip to San Gimignano also. Or rent a car for a few days and stay at an agritourism. Just be aware that you have to leave the car outside the historic centers or incur fines of hundreds of euros.
Cinque Terre I'm with Angela. Wrong time of year, forget it.
Bologna is a good base for day trips to Padua, Ravenna, Ferrara and a lovely town to boot. Ferrara might be a good base too.
Venice 3 nights, 2 full days. That's the minimum. Don't plan on day trips from Venice.
Verona is one of my favorites. It's between Venice and Milan, not a day trip from Venice. Also, it is best experienced as an overnight stay.
Milan Plan to spend one full day to see the sights, possibly two. Then you never have to go back.
You will have to cull from this list. There are several more than your 21 nights and days (don't count Dec. 6 at all - with an early afternoon flight, you won't have time for more than a leisurely breakfast before heading to the airport). Figure on 1/2 day used for each hotel transfer - more if you include the Amalfi Coast. Even if you have a car, you still need to pack/unpack, check in/out, and park the car.