ZTLs in Florence, Renting Car for Tuscany, Return in Florence for Amalfi

We are picking up our rental at the airport, about 3.5 miles or so from Firenze, to avoid the whole ztl thing. We have about 3-4 day trips planned over 6 days in Siena, We were in Florence a year ago for 9 days. Its not just the ztl's, its also having to stay out of the bus lanes, speed cameras that track cars from point to point to enforce speed limits. Hence the airport site.

We really loved the brief time we spent in Siena, the Cathedral is incredible, from the outside to the inside and the floor of the cathedral, it is jaw dropping. Hence we are returning there this time. If you haven't been I recommend it. Siena is beautiful and worth a visit.
https://www.discovertuscany.com/siena/tourist-info/parking-lots-in-siena.html

If you drive the Val D'Orcia you would be seeing some of the best scenery that includes the town of Pienza.
https://www.discovertuscany.com/valdorcia/

This web site has a great assortment of information including a suggested drive through the Val d'Orcia. among others. The link takes you straight to the recommended route with lots of details.

http://www.slowtrav.com/italy/tuscany/sd_driving_tuscany.htm

• international driving permit, get at triple A before leaving home, about $25 last I heard. One needed for each driver listed on the rental agreement.

-check routes and planning at https://www.viamichelin.com/

I understood from another discussion that cars can be dropped off in Orvieto, just not rented there. but recheck that. I am sure someone here will comment on it. Its about 2 hours from Siena.

If you pickup the car as you leave Florence, your rental agent can give you directions out of Florence avoiding any ZTL's. You might have more chance of getting an auto tranny if you picked it up at the Florence Airport, however there may be additional charges, taxes and/or tolls when renting at an airport and often train station location.
The drive from Florence to Sorrento is between 315 and 350 miles--depending on whether you are game to face Rome's traffic. After driving all the way down there, I'd hate to think I had to drive all the way back to Florence.
You might consider finding a city south of Tuscany where you can turn the car in and catch at train to Rome and to Naples and the Amalfi Coast.
A favorite car return is in Orvieto at the Hertz agency--across the street from the train station--about 90 minutes south of Siena. I don't know if Kemwel has any facility in that city. It's 70 minutes by local train into Rome Termini from this fabulous hill town.
As far as rambling through the countryside, we loved Volterra, Certaldo and San Gimignano. We found Siena to be very congested and parking very difficult to find. And we also loved Orvieto.
Good luck to you on your odyssey.

1. I'm assuming that by Labor Day you intend International Workers' Day (May 1). The ZTL doesn't operate on that day and is accessible to all, so you would have nothing to worry about. On any other day getting out of Borgo Ognissanti is easy. Just drive in the only direction possible (Borgo Ognissanti is a one way street heading west). Go straight all the way to Piazzale di Porta al Prato where you are out of the ZTL. No cameras on exiting the ZTL.

2. I would definitely return the car in Sorrento, near the Amalfi Coast (actually I'd keep it for the Amalfi Coast too, because I like to have a car under my arse at all times). Below you are talking about going all the way to Montepulciano. Why would you want to back track all the way to Florence? You are already half way on the way to Rome.

3. I specifically ask for manuals (even when I buy cars in America, which is increasingly difficult) so I'm not sure about this. Borgo Ognissanti location is heavily used by Americans (the only ones who ask for automatics) so the chances of them having automatics are high. But the deeper question is: why don't you learn to drive manuals? Automatics are for pussies.