I have lived in Milano for almost a year and will provide my best advice having been to all of the places listed. Here are some area I will comment on:

1. There is a baggage locker in Verona station, its cheap and safe.
2. 5 Days to Explore Venezia IMO is going to be way too much time. You can really see all of stereotypical (canals, etc) Venice in 2 days.
3. The train from Venice to Cinque Terre is going to take sometime, I am not sure you will really have enough time to have an "Afternoon" that day to explore a whole lot, just FYI.
4. Best advice for Roma: DO NOT try and do Roma in 1 day; its not possible. It appears you and I have similar travel styles; we like to get in, see what we want to see, and get out. I can tell you I have tried Roma in 1 day twice with 2 different friends of mine visiting from the states and it just is not possible. You need 2 days at a minimum. It just takes time and things close around 5pm so you inevitability will end up missing some things. Also, the major attractions in Roma (Collesium, Sistine Chapel, etc) have long waits, so this quickly offsets the time you truly have to see the city in one day.

In closing, I would take at least 1 of the days you set aside from Venezia and give it to Roma. Trust me, you will not regret it. It wouldn't hurt to locate to FCO the night before departure either if you value a cheap taxi ride and a good nights sleep.

In Boca Al Lupo!
Ciao ciao