We're very glad you found the forum and are finding it useful! We hope to be able to answer the questions you've had in planning your trip, and hope you'll also share with us what you think about and learn from your experience in Italy .
Right now I'll answer just one part of your questions which influences some of the others - the trains from Borgo San Lorenzo to Florence are all regional trains and as such cannot be reserved and purchased ahead of time on the web. This is no big deal, all regional train tickets are NOT sold for a particular trip - they are valid for 2 months from time of purchase, and are "used" when right before you board the train you stamp the ticket in the machines along the tracks. Trains and buses all work this way in Italy, so make sure to remember: stamp your tickets before getting on a train and right after you get on a bus.
Basically, you can buy your train ticket 15 minutes before you plan on getting on any regional train, just give yourself the time to buy tickets and get to the right track to catch the train you want.
As such, if you search on the Trenitalia website from Borgo San Lorenzo to Firenze Santa Maria Novella, you'll see there are several trains in the morning. I've copied just a few of the more direct ones. The longer trips are because they take a more roundabout route to get to Florence, these are trains used by commuters into the city all around the area. The shorter trips are the ones that run alongside the "Faentina" road and that is why it is called the Faentina line.
On the other hand, you can also choose to drive into Florence if you don't feel the schedules are right for you (you could do two day trips into Florence and try both one on a day when your times are more flexible). You would have to take the Faentina road (Via Faentina) from Borgo S.L. and soon after you enter the city boundaries you'll find a parking lot on the right. I highly recommend you leave the car there and take the bus into downtown - here is a map of the parking lot at the Salviati station (click on satellite view to see it better) so you can see where it is in relation to the road. I don't tell you about the station there because I really don't think trains from Borgo S.L. stop there. But it is a great lot to leave your car, many people do that who don't want to deal with driving in Florence.
Hope this is useful so far!