I am so excited
Venice airport= Marco Polo, VCE
Rome airport= Leonardo da Vinci-Fiumicimo, FCO
you have 12 nights
1 night in Siena is fine- arrive early- I believe you will be there when the Duomo floor is uncovered- how lucky!
I'd give that extra night to Venice
4 nights in Florence does give you time for a day trip- there are lots of places/ways to do that- whether on your own via bus/train or with a guided tour.
Walks of Italy offers a few- this one is Lucca, Pisa, winery lunch
Are your hotels already booked? If so- Are they refundable?
Make sure you have air conditioning
Your Venice hotel is right on edge of S Marco Plaza, looks nice but it's super busy around there and looks like you'd have to drag your bags quite a ways, there are no canals nearby for a water taxi.
We have stayed at Hotel Ala twice now and would always choose again when we return to Venice. The location is excellent, in a quiet campo, steps from S M Giglio vaporetto stop with NO bridges, steps or canals to cross. About a 5 min walk to S Marco. Great bkft and nice roof deck.
Florence- hotel location is great. I have no hotel to recommend as the place we stayed was lovely but apparently bad reviews since!
Rome- also good location but on a very busy, noisy intersection.
I highly recommend Albergo Cesari or Hotel Due Torri. Both a bit more into the pedestrianized area, very central. We walk everywhere from there.
Transportation- once you settle itinerary- come back and ask how to get to and from- all the info is here.