Just to confuse you further, I may as well throw my 2¢ into the discussion......
Where are you flying from? I'm assuming you've allowed for the fact that you'll usually arrive in Europe the day after you depart and the last day will be spent on the flight home. Have you purchased your air tickets yet? Although round trip flights to Milan may be cheaper, you also have to factor in the cost of rail travel from Milan to Venice, which could range from about US$55 to $95 PP. Using open-jaw flights (ie: inbound Venice, outbound Rome) would be a more efficient method of travel, and would allow you to maximize your sightseeing time, and may ultimately be cheaper when all is considered.
This is one suggestion on how I might structure the trip you described......
- Day 1 - Flight to Italy
- Day 2 - Arrive Milan, take train to Milano Centrale and then Venice (that trip will take ~3-4 hours, possibly direct or with 2-3 changes, depending on what time your flight arrives and which trains you connect with)
- Day 3 - Venice
- Day 4 - Train to Florence (about 2 hours), touring Florence
- Day 5 - Florence
- Day 6 - Florence, day trip to Pisa
- Day 7 - Train to Assisi (2-3 hours), touring Assisi
- Day 8 - Assisi
- Day 9 - Train to Orvieto (2-3 hours)
- Day 10 - Orvieto (day trip to Civita if desired - I believe a visit to Civita is worthwhile as it's somewhat unique - however you'll need to plan carefully when travelling by public transportation)
- Day 11 - Train to Rome (~1.5 hours), Rome touring
- Day 12 - Rome
- Day 13 - Rome, day trip to Pompeii (~3-4 hours each way)
- Day 14 - Rome morning, afternoon train to Milan
- Day 15 - Flight home
I suggested two nights in Assisi as you really seemed to be interested in that, and one night really doesn't allow much time. I try to avoid one night stops as typically that means arriving mid-afternoon and only having the afternoon and evening to see the place. Hotel check-out is usually about 10:00 to 11:00 so that won't allow much time for sightseeing the following morning. Staying two nights allows one full day for sightseeing.
You'll have to do some research on rail travel in Italy, as there are potentially expensive "caveats" to be aware of. Also, when buying tickets you'll need to know the Italian names of the stations you'll be using, since most cities in Italy have more than one.
Good luck with your planning!