Some Rail Confusion

I would look up the advance purchase prices for your three rail trips on the Trenitalia site. You can look up both second class and business class prices. These prices include all reservation fees, but, like airline tickets, are specific to one train and once you buy you cannot change your mind.
Then you can compare the cost of normal tickets (advance purchase) with a pass plus separate reservation fees.

Yes the 1-facing-1 across a table seats are good for a couple. That implies 1st/Business class.

I just noted that at age 61, I might qualify for a Carta de Argento which (for 30 Euro) would get me 15% off my ticket, which might be worthwhile and change the math as well.

While you are checking the real prices of tickets - because you will be buying reservations on the Trenitalia trains if you have a pass and once you have that reservation it is for one and only one date and time, if you want a different one you gotta buy another - so you will be seeing silly cheap prices, also check Italo Treno, the newish private (don't take passes) (but have even cheaper and nicer trains) and which I prefer. They compete on all 3 legs you mention.

Note that nobody provides fancy trains, or at all fast trains, between Napoli Centrale and Sorrento. It is a taxi, a boat, a prearranged ride, or downstairs to the colourfully graffitied, slow and second class only private commuter train the Circumvesuviana. It is super cheap, it takes no passes, it is private so not ticketed through Trenitalia and takes about an hour, making stops every 3 or 4 minutes.

October can be a really nice time to travel.

What do you mean by "some comfort"? The Italo has leather (hand stitched corinthian? nah) seats in 2nd class and I find them really comfy, and I'm a pretty big guy. They are definitely comparable to business class on a plane. But with more room.

To be honest its not really worth paying anything extra than the lowest fare. These journeys are a few hours each and you will both be sitting together. Walk to the cafe/bar car to stretch your legs. The journey will be over before you know it.

I don't think you'll need a pass for these shortish trips, get your first-class tickets 120 days ahead and you may find them cheaper than full-price second class tickets.

You probably don't want the Circumvesuviana train if you are looking for comfort, get a private transfer directly from the train station to your hotel.

If you "want to travel in some comfort," I recommend that you always travel in the top most confort you are willing to pay for. And always grab the comfort when you can. You will need that comfort to take you through or avoid the rough patches out there.

I never mind paying full fare for first class if available. The alternative is difficult. Regional trains fill up fast with locals, students, soldiers, and workers. I've never sat down on a Regional train.

Last year I was on a Regional from Napoli to Roma with a change at Cassino. The elevators at Napoli Centrale were all broken. A bunch of old people were left standing in the vestibule I was in. My lower back could not withstand the lurching. At Cassino I had to navigate mile-long stairways (no elevators, no ramps -- very common in Italy), and then wait for the train on gravel in weeds trackside (no platform, no seats). At Roma Termini, the train had a staggered stop, giving me a one - two mile walk to the station. It was Midnight. Train trip from Hell. I had caught the train at 5 pm at Napoli.

So if first class is available I'm on it. I travel with only one carry on bag weighing a total of 15 pounds. That enables me to compete with the locals for a seat on a Regional I could not avoid. It enables me to climb and descend those humongous stairways at stations. A staggered stop is almost tolerable pulling a light bag.

Travel in Italy is work, so pay for the comfort when you can.

Sandra, that is the worst luck I've ever heard of! Good that you are traveling light. I have occasionally had to stand on a train, you are right, it takes a toll on your back if you have to stand too long.

If you do use a 1st class rail pass to travel on a Frecciarossa train, you would be making your 10-euro seat reservation in "Business" class. On www.trenitalia.com, you can start a ticket purchase, then choose "other offers," then service=Business and Offer=Global pass, and enter the pass number, and then choose seats.

You can also book reservations at a train station, or a travel agency in Venice, such as when you have the pass activated.

If you decide to go with the advance-discount tickets, they usually go on sale about 3 months in advance of each travel date. Also, changes are expected to the Frecciarossa from June 14, if the new version 1000 goes into service.

Be aware that there is some controversy about the value of "first class" (or whatever it's called now) on Italian trains. Some feel they are always worth it, others feel there is not enough difference from "lower" classes to justify the cost. My brother was in the latter category; he took second class for most of his trips (Venice to Bologna and Bologna to Florence) but first class from Florence to Rome. Having read the various opinions, I was very curious what he thought. Before I could even ask him, I got a profanity-laced invective about what a ripoff first class was! But, as I said, others do say it's the only way they will go.

The tip above is worth repeating; if you book early enough and can accept non-refundable and non-exchangeable tickets, you can often get first class for little more than second class, a win-win.

When we traveled to Europe in March, we took 2nd class for everything--Paris/Lucerne, Lucerne/Milan, Milan/Florence, Florence/Salerno. Very comfortable, very inexpensive. We booked each leg separately about 75 days ahead. Yes, we were regulated to that train on that day, but my feeling was that if for some reason we were unable to travel that day, we'd be out so little money that it was worth the gamble.

But...had we taken this trip in high season, I certainly would have investigated booking 1st class. In March the trains were half to two-thirds full at most in 2nd class, plenty of overhead room for bags. That might have not been the case in July.

Just sayin'...