First time to Italy for 19 days

Why not base in Naples? It's going to have a lot more to offer in November than Sorrento. Check your flight options out of Naples v Rome and then decide. You could reverse the Rome and Naples stays if the Rome flight is better.

I think your plan of Venice, Florence, Siena, Rome is excellent. Personally, I'd spend all 19 days in those locations, taking a day from Siena and adding all the additional days to Rome. There is so much to see in Rome, and there are a number of day trips (Ostia Antica, Villa d'Este and Hadrian's Villa at Tivoli, Naples, etc.) that are quick and relatively easy from Rome.

Is there a particular reason you want to visit Naples? The city itself can be reached in just over an hour from Rome. If you're really thinking about Pompei, that's a longer trip and makes for a long day trip. The Amalfi Coast is a bigger time commitment and at some point, you can't reach everything by train.

Give us more information about your interests.

For your flight, check all your options for departure, and end in your departure city. If you're ending in Naples, you'll definitely want to be IN Naples the night before.

On our most recent trip, we flew into Naples and out of Rome. It worked well. We liked Naples, we were there 4 nights.

Are you visiting Paris and other areas in Europe before Italy?

Coming from Seattle, I think you’ll appreciate the urban grittiness of Naples. I thought it was pretty rad, and really the tourist’s Naples is fairly compact. Sorrento’s main attraction is more as a launchpad for sights around Naples and the Amalfi Coast. It’s also just a “nice” town free of stress or complications. For this trip, I’d just stay in Naples. See the Archeological Museum, do a day trip to Pompeii, Herculaneum or both and just explore the city a little. You’re going to enjoy experiencing the differences between all these cities.

Where to fit it in though? You’re most likely to find your flights out of Rome. If that’s the case, I’d suggest taking the train right from Tuscany to Naples, saving Rome for last. This might seem like a longer day on paper, but the intercity train is a smooth relaxing ride. This will likely include changing trains in Rome, but it ought to be easy. There are frequent trains between the two.

You've gotten some great advice, and your itinerary looks quite realistic. I too would take at least a day from Siena, or maybe 2, and add them to Rome. An Italian friend of mine says you can live your entire life in Rome and not run out of things to do!

I enjoy Naples, but you may not, especially as it's your first time to Italy. Be realistic. Naples is more chaotic than Rome and isn't everyone's cuppa, which is why many people stay in Sorrento, in addition to easy access to the Amalfi Coast. This video cracked me up, just a reality check on Naples, Instagram versus reality! https://youtu.be/6AFwP5zrKL0?si=cgV3rGi_Dgz8_uY5. As another poster said, you could skip Naples and spend your entire trip in the first 3 places, the time will fly!!

You're going late enough in the year, you could easily do a day trip to Capri, which is gorgeous but busy during holiday season.

What would you like to see in the area? If your primary draw is Pompeii, you can actually stay at the Scavi in a nice hotel recommended on the RS Forum, (about a 20 minute walk to the main entrance if meeting a tour, otherwise entrance just across the street.) AND you can take the regular train rather than the little commuter. I was pleasantly surprised at how much more inexpensive it was to book over NYE than Naples or Sorrento! http://hotelforumpompeii.com/en/

And, when I leave and fly back to Seattle, it seems like a difficult task from either place, so will I need to book multiple flights?

You need to make a single booking that will involve at least 2 flights, Probably better options from Rome than Naples.
As already suggested , arrange to have Rome be your last destination, likely will make finding a flight home easier.

I would suggest one less night in Siena and add it to Rome

If Naples is what you want to visit, Sorrento is not an ideal place to stay.

There are definitely more flight options out of Rome, but Naples does have a number of flights. For us, looking at the schedules, it made more sense to fly into Naples and out of Rome because the Naples flight to home, were rather early in the morning.

I don't really think you need the AC in this trip, but if it's important, put it at the beginning. As you get close to November, the weather will get rainy and cloudy, much less ideal. The AC is not an indoor area. You'll also need to check the dates the ferries reduce their schedules.
I think your original four destinations are great. Enjoy your trip!