It seems to me that you are planning a vacation that you and your friends have yet to finalize. That's good - planning and re-planning can be, and should be, part of the fun. I would suggest getting out your RS guidebooks and over pizza and wine (Italian, of course), the 4 of you come up with a list for each city and prioritize it. How much time do you need? Did you plan for the 'riposo' hours in the afternoon when many places close up. (Sorry Rick, but it always takes me longer to visit a site than you do.) Getting tickets for museums that have secured entry times will help you save some time since lines can be long in Rome and Florence. Make sure you check train schedules to see how much time will be lost on travel days. Don't forget to include packing/unpacking checking in/checking out of hotels. Some days can become quite short!
No Venice? I personally would take away Cinque Terre and Milan and give those days to Venice. And take a day or two from Siena too. Venice is not a place to go and 'see things'. Venice is the 'thing' itself and you enter into it. Walking out of the train station immediately puts you in a totally different experience. It's different than Rome, Siena, Florence - it's not art in a museum, it's art you wander through. I never spend less than 5 nights in Venice every year for more than a dozen years now. Once you've seen the major sites (ok, there are those), just turn left at the next alley and try to get lost! A whole new appreciation of Venice is there for you to discover.
Assisi is another matter. There is some magnificent art to be found in museums and churches (not just the 3 main ones). Don't forget to discover the ancient Roman ruins under the Piazza Comune. And also the more ancient Etruscan ruins under Santa Maria Maggiore. There's a lot more to Assisi than St. Francis and St. Claire. Assisi is also a wonderful place "to be". Once the tourist groups leave, late in the afternoon, the true benefits of Assisi become available. Go to Santa Chiara (St. Claire) and listen to the cloistered nuns sing evening prayer (6 o'clock). Walk through the city's various levels (remember it's built on the side of a mountain) at dusk and late at night to enjoy the cool evening air and the views. No wonder Francis spoke so much about 'peace'. Full truth: I spent 16 summers in Assisi as part of an international music festival.
Happy planning. I know this site will give you plenty of help and ask you plenty of questions to nurture your dreams. Remember: you deserve this!