"but would taking a ferry from Naples to Sorrento be a viable option with luggage?"
That wouldn't be my first choice (in fact for me it would be a darn nuisance). You'd first have to take a Taxi from Napoli Centrale to Molo Beverello or wherever the Ferry departs from, which is going to add time and cost.
The first part of your trip is easy, train from Arezzo to Napoli Centrale. The quickest solution seems to be a departure at 08:34, arriving at 11:28 (time 2H:54M, one change at Roma Termini). The later departures take considerably longer. When you arrive in Naples, you have several choices.....
- Circumvesuviana (cheap but a bit "colourful" as it's a commuter train.... watch your luggage carefully during the trip)
- Campania Express (more comfortable as it's a tourist train, but whether it's still operating "later this fall" will depend on exactly when your trip will be taking place).
- Private transfer (this will be the easiest but also the most expensive solution.... if your budget allows, arriving in Sorrento via air conditioned vehicle will certainly be the easiest and most comfortable choice.... I believe this service is still listed in the guidebook and it doesn't cost anything to get an estimate - http://www.monettitaxi17.it/English.htm ).