Best itinerary

Sounds like a great itinerary however I would not change hotels for each town. I would pick a central town and stay there the entire time. You didn't mention whether you'll have a car or not but it's very easy to take day trips to 1-2 towns per day while staying in a central location. You will find it very exhausting to pack, unpack and check into and out of a different hotel every day. At the very most I might stay in 2 different locations for the 7 nights but not a new location every day. We stayed in a wonderful Agriturismo for our first trip to Tuscany, had a car and drove to various towns every day and then returned to the farmhouse every evening for a wonderful meal and lively conversation with other couples also staying there. It was a great experience and we plan to repeat it this coming August. Our Agriturismo was in Siena and we easily visited all of the towns you mentioned as a day trip with the exception of Cortona and Assisi which we plan to visit this year (again from our base in Siena). Just my two cents but I really think you'll have a more pleasant experience by not changing hotels every day. On our first trip to Italy, we switched hotels and locations every 1-3 days and it was too tiring and time consuming. Have a great trip.

The places you want to visit are small towns (except for Siena, which is a small city) therefore they are most efficiently done by car if all you have is 7 nights. Consider that you will probably have (want) to spend the last 4 nights (or at least 3 at the very minimum) in Rome. I would rent a car on arrival and head to Tuscany/Umbria on the day of arrival. If your flight arrives late and/or you are too tired to drive, just find a hotel closer for the first night. Then spend 3 or 4 nights in a central location to visit what you can. The last day in Tuscany drive early to Orvieto, where you return your car (only Hertz is in Orvieto). Visit Orvieto for the day, then later in the afternoon take the train to Rome where you will spend the last 3 nights. I don't think you have the time to visit all the places you mentioned, but if you plan to spend the nights in different places each night you will have even less time for visiting, changing hotels takes time. The towns you mentioned can be driven to in less than 90 minutes from a central location somewhere near Siena. If you don't plan to see Rome at all (maybe you have visited it in the past) then you can stay longer in Tuscany/Umbria. In this case you still need to spend at least the last night in Rome or somewhere close to the airport. At any rate it is more efficient to visit all those places from a central location because they are close enough to one another.

Logistics for Volterra are difficult given your limited time and long wish list. I would limit myself to the following if travelling by train.

Assisi (1 night)
Cortona (1 night)
Montepulciano (2 nights) via Chiusi
Siena (2 nights)
Orvieto (1 night) via Chiusi

Taking daytrips from two locations will save you a couple of hours each time (traveling unencumbered versus packing, checking out, checking in, unpacking). But you will have roundtrip travel time for those days. I would still use just a couple of bases, though.

Based on the marvelous feedback you get on this site, there are two great ways to visit Tuscany.

1. Stay on the northern side in Chianti for a few days - visit Radda, Greve, Volterra, and San Gimi, etc. Then stay in the south - visit Montepulciano and Pienza, Todi, Cortona, etc.

2.Stay in a great agriturismo the whole time and just drive - like Cretaiole, which is excellent.

We have done the "stop every night" thing, and even though it is an adventure, we did get tired of moving every day….

I don't know about other people but when travelling I usually aim to get a few days wear from items in my carefully chosen wardrobe. So a single night stay often involves no unpacking and repacking, just the need to pull out my toiletry bag and fresh underwear.