Things to book beforehand in Italy

Looks like you got it figured out pretty well... Unless you are going to borghese gallery in Rome. You must have reservations for that.

Book the trenitalia trains in advance to save money IF you know exactly when you will be going from place to place, as these are non-refundable. The regionale trains you can't book more than 7 days in advance (i.e. Florence to Pisa), and there is no need to even do that as they rarely fill up.

Few people on this website will know anything about using Indian credit cards. I know my hubby has some issues when he visits India and uses American credit cards, particularly for booking trains. I don't know what kind of issues you will have in Europe. Sorry.

If you get the Florence Card it will get you in the short line for Uffizi and Accademia; Roma Pass will get you into the Colosseum quickly. You need to see if they are good deals for you based on what else you want to see. Can be pricey.
Get your hands on a good guidebook to see what sights would interest you. RS Italy is what we used.

You don't need to book ahead for entrance to Pompeii. To avoid a ticket line at the main entrance, you can also buy tickets at the "Info Point" kiosk when you exit the local Circumvesuviana train. You also don't reserve for San Marco. You might find a shorter line at lunchtime or when it's raining. If you have a bag that's big enough that it needs to be checked, then you go straight to the bag check and skip the line. (These tips come from Rick Steves' Italy book.)

There's no deadline to buy train tickets. Train tickets between the big cities will be sold for reserved dates, times, and seat assignments and are available now online, or you could reserve a few at a travel agency in Venice.