Angela,
To begin with, I would not recommend speeding up your Itinerary. As you noted, it's important to include transportation times between locations, which is usually about half a day and in some cases the better part of a full day. That means you won't have as much time in each location as you might think (ie: your 3 days in Florence will only be about 2.5 days).
Does the 21 days include your two flight days? You've been there before, so will be aware that you'll arrive in Europe the day after you depart home, and the final day will be required for the flights home. Also, you'll likely be jet lagged for a few days, especially coming from the west coast (that's usually the case for me).
The order of your Itinerary seems good, and it should easily be possible to include a visit to the Cinque Terre as well, but will require some adjustments in other parts. There are two ways you could do that.....
- Add time in Florence and visit the C.T. as a day trip. It will be a bit of a long day, but if you leave early and return late, you'll be able to see the five towns. I've made that trip, and it's not really that difficult.
- Spend a night or two in the C.T. which would allow you to have a more relaxed visit there and perhaps some beach time. That's a great place to "recharge batteries" if you're exhausted from travelling in other areas, and you can do as much or as little as you want. However, if you decide to do that I'd recommend changing the order slightly and travelling Rome > Cinque Terre > Florence > Venice, as it's a more efficient route. One other suggestion - it would be a really good idea to book accommodations WELL in advance. I normally stay in Monterosso, which is the largest of the five towns so has a greater number of hotels, restaurants, etc. as well as the nicest beaches.
Flying from Paris to Rome is indeed the best idea. You might have a look at easyJet flights from Paris / ORY to Rome / FCO (flight time ~2 hours). A few points to note with those flights.....
- Those booking well in advance get the cheapest prices. They often use a "sliding scale" with prices increasing as the flight fills.
- Most budget airlines offer different types of tickets, but the cheapest ones are usually non-refundable and non-changeable, so choose carefully when you book.
- Be sure to read their Terms & Conditions carefully, especially related to luggage. If you think any checked bags might be over the weight allowed, it's cheaper to pay for the next weight tier at the time of booking. If they find it overweight when you check it, the cost will be much higher.
- NOTE that easyJet only allows ONE carry-on item per passenger, of the approved size (they don't care about weight as long as you can lift it). This doesn't mean "one plus a personal item such as a purse or laptop case", it means one only!
- it's relatively easy to get from Paris to Orly Airport (there are several methods). One of the easiest is to take the RER "B" to Antony station and then take the OrlyVal people mover to the airport.
All of your trips are easy by train, but one to note is the trip from Venice to Salzburg. That's easily done, but in order to keep the travel time to a minimum, I'd probably use a departure from Venice Tronchetto at either 09:20 or 11:20, arriving at 15:48 or 17:48 respectively (time 6H:28M, 1 change). This solution uses a Bus from Venice to Villach for the first segment, and then train into Salzburg. Both segments require compulsory seat reservations.
What a wonderful trip that will be, both to celebrate your anniversary as well as travel with your daughters while they're still at home.
Good luck with your planning!