Hello Everyone, Hopefully you will be able to help me

Here is our itinerary:
May 8th Arrive Venice in the morning (try to see as much as we can in venice)
May 9th Leave on cruise in the afternoon
May 16th Arrive early in Venice -
Leave to Rome and stop in Pisa and/florence and spend the night ? 
May 20th Fly back home from Rome
We are trying to plan as best as we can the journey from Venice to Rome by train with the little time we have available (3 1/2 days). We though of maybe going to Florence and Pisa, and we want to have at least 2 whole days in Rome. What would be the best way?
1) VENICE-
FLORENCE-PISA-ROME - saturday leave for florence and stay there for the night, the next day go in the morning to Pisa and from there leave for Rome in the afternoon?
2) VENICE-
PISA-FLORENCE-ROME - staying in Pisa?
3) VENICE-FLORENCE-ROME same day and have more time in Rome?
4) How long does it take from Florence to Pisa ? how can you get there?
5) Regarding the tickets, if I do this stop could I get a train ticket say from Venice to Rome allowing overnight stop in Florence, transfer to Pisa and go from there to Rome? should I buy separately?
6) Is there any other city on the way you suggest would be better to visit maybe by not going to Pisa?
sorry if its too many questions

Thank you so much!!!!