The Naples-Salerno train stops at the main Pompei (town) station rather than the one (Pompeii) near the excavations. You'd have a bit of a walk; you can check Google Maps to see how far it is. If you decide to use Salerno as a base, you should consider taking the daytrip to Naples from Salerno rather than from Positano. I imagine the transportation cost to Naples from Salerno would be lower (I haven't verified that) and the travel time certainly would be shorter. And switching one hotel night from Positano to Salerno should be much, much cheaper. In addition, I've read that bad weather can prevent ferry arrivals and departures at Positano; that wouldn't be an issue if you daytripped from Salerno.
It's highly unlikely a rail pass would save you money. If you're willing to commit to your travel dates and buy non-refundable tickets well in advance (yes--that's riskier than usual these days), you will pay much, much less by simply purchasing point-to-point tickets. If you're uncomfortable with buying tickets far enough ahead of time to generate substantial savings, you can check the likely cost of point-to-point tickets by pricing out your trips as if you were traveling this week. Use the Trenitalia website: https://www.trenitalia.com/en.html. You'll need to use the Italian names for the big-cities: Venezia S. Lucia, Firenze (Tutte le Stazioni), Roma (Tutte le Stazioni), Napoli (Tutte le Stazioni). Compare that cost to rail passes.
The company Italo (https://www.italotreno.it/en) also runs trains on the line from Venice through Florence and Rome to Naples. I've read that its fares are sometimes a bit lower than Trenitalia's. When the time comes to actually buy tickets between the big cities (if you don't decide on a railpass), it would be smart to check Italo as well; you might find discounted seats there after all Trenitalia's discount options have sold out.
Since I can fly non-stop to and from Rome and it would typically be much cheaper for me to depart from Rome than from Naples. It's a balance between dollars and convenience. Departing from Rome might require you (depending on departure time and tolerance for risk of train disruptions) to spend your last night in Rome. However, if you're already planning to relocate from Positano to Naples the night before the flight, going up to Rome really wouldn't be much more burdensome.
