It's true that trains don’t always use the same arrival and departure tracks and that you have to check either the monitors or the big board. On the other hand, Trenitalia publishes the list of all intended arrival and departure tracks, by station and train number.
The train from Turin usually stops at platform #11. The 14:14 trains to Spoleto and Foligno usually depart from platform 14, ten meters away from platform 11 if you start walking towards the train engine before arriving in Florence.
Do you have to validate online the Regional train's ticket? It should be explained in the email from Trenitalia.
This station is a terminus, meaning everyone will be getting off the train.
No, it doesn't mean that. In fact the 14:04 Freccia #9535 from Turin ends its run in Naples, not in a middle sized city like Florence.