Car Rental Pick Up/Drop Off

I rented a car from Europcar near the RR station (not the easiest office to find) and drove out of Naples at about 9:30am and it wasn’t bad at all. Regular “big city” driving but uneventful. I don’t know about driving into the city because I returned the car at Fiumicino airport.

Driving in Naples IS insane.
However, it is only about 1/2 mile from the start/end of the A3 highway out of town to the exit/entrance of the Piazza Garibaldi parking garage, where I presume you'd get the car. So you should be OK: 1 left turn, 3 right turns, 2 left turns, and you're out of town.

How about renting in Naples and dropping off in Salerno? is that an option?
Or renting at Naples airport, which is very close to the city center without any of the traffic stress, and also dropping off in Salerno - or at the same location?

Your logic looks spot-on to me. Would suggest you add this info to the mix:

When exactly (how far in the future, and also day of week/time of day) is your trip and your expected pickup/drop-off times?

In general, I prefer to lower the stress level around car rental tasks, so that alone would make me lean away from Naples toward Salerno.

Late night, I'd be (even more) inclined to avoid Naples (perceived safety concerns). Mid-day it might not matter much. Also check the times that each office is open.

Nobody knows how the pandemic will progress/recede and what impacts that will have. Personally, I'm guessing (cautious optimist here) that in a few months, things will be noticeably better, so if your trip is not very soon, I'd be less worried about delays caused by COVID/staff shortages than I would if your trip was coming up soon. That's just a guess though, things could go either way, of course.

Definitely pick up and return at Salerno.
When looking for trains use the fast trains that go thru Napoli Afragola station. Travel is generally 20 minutes more than trains from Napoli Central.

Salerno pick up will enable you to use the road 847 that goes thru Potenza to Pulgia nice road, not as trafficked. Use Via Michelin for directions. Drive will take longer than they state.
If you decide to start in Naples, the airport location would be less stressful. Taxi or Alibus to the airport.

We rented a car at the Naples ferry terminal. It was easy to get on a major road heading south. We would rent at this location again. We then returned the car to FCO airport.

We dropped a car off at the Naples airport, that we had rented in Umbria, very easy to do. The airport is up the hill a short distance from the city proper, just off of the autostrada. Pick your car up at the Naples airport, jump on the highway and head to wonderful Puglia.
Or, when we went to Puglia we flew from Rome to Bari, fast and inexpensive. They offer flights from Naples to Bari too.

We recently (in late Oct.) rented a car at Avis in downtown Salerno, drove up to Tuscany, keeping the car for a week before returning it to the airport in Rome. The "one way" fee to pick it up in one city and return to another wasn't bad at all. Only 48.60€. I believe these one way fees are much lower in Europe than what you'd see in the U.S., as long as you are picking up and returning in the same country. We got fabulous service at the Salerno downtown Avis location. We arrived about a half hour before our car reservation time, and the young man behind the desk needed that time to complete our paperwork and get our car ready. He allowed us to leave our bags there, secure under his watch, and we went next door, where a cute little cafe had outdoor seating, the perfect place to have a cappuccino and pastry while waiting. The Avis agent upgraded us a bit when he saw how many bags we had. The original car had a trunk and he wanted us to have a hatchback to make it easier to fit in all the bags. He loaded the bags into the car (4 passengers) so we could see the best way to fit them all in.

I'm late to respond because it has been a busy week - but thank you so much for all your insight and experience. It is super helpful!