Drop the CT, add time in Lauterbrunnen area IF YOU ARE HIKERS. It's good to stay there 4 nights so that if there are weather problems, you have a better chance of some clear weather, and if you are hikers you are going to LOVE it. Otherwise add an extra night to Roma. The itinerary below short-changes Roma, but at least you get more than 2 nights in most places. Also, it is not sacrilege to skip Firenze. We didn't go there until we'd taken 2 long trips to Italy and lived here for over a year. You could do the CT instead of Firenze or Venezia for that matter if you are more outdoors people and less art museum lovers. Or you could skip Firenze and add the time to Roma allowing some day trips while there. The point is you need to have fewer transfers, more time in each place to enjoy. And beware the costs in Switzerland! It is very expensive. Luckily the cost of Italy is now much better thanks to the exchange rate!
Use Rome2Rio.com to get an idea of intercity travel options.
Day 1 - arrive Geneva, assuming you arrive by Noon, go immediately to Montreux as it is only an hour's trip, 2 nights
Day 2 - Montreux
Day 3 - travel to Lauterbrunnen Valley, 4 nights
Day 4 - Lauterbrunnen
Day 5 - Lauterbrunnen
Day 6 - Lauterbrunnen
Day 7 - travel to Venezia, 3 nights
Day 8 - Venezia
Day 9 - Venezia
Day 10 - travel to Firenze, 3 nights
Day 11 - Firenze
Day 12 - Firenze
Day 13 - travel to Roma, 3 nights
Day 14 - Roma
Day 15 - Roma
Day 16 - fly out