Hello jjec,
Offhand I think it is too many places spread over too large an area.
Definitely 3 nights in Venice since one of those nights is a flight day.
Bologna for 3 nights is a good amount of time.
Spending the time to cross most of the country to spend two nights/one full day on the Ligurian coast seems like a lot of travel without a solid plan. I'd either commit to spending the time there or do something else. You could do the Ligurian coastal towns from Levanto or the south side of the La Spezia bay with a side trip to CT or commit all the time to exploring CT.
Orvieto is well past San Gimignano and getting back to it would mean travelling back to Tuscany for many hours and probably require a car. Personally I don't think SG is really interesting enough to backtrack for or to stay in. If you wanted to do 2 nights in Siena/Arezzo and 2 nights in Orvieto that would make a little more sense. But either way if you want a taste of Tuscany do this before heading to Orvieto which is well on the way to Rome.
Keep in mind that 2 nights is only one full day in a place and personally I don't ever advocate staying somewhere for one night until it is for transportation or logistical reasons.
Just my thoughts, hope that helps. Have a great trip,
=Tod