Hey Mary, every poster is different. I lean towards "teaching a woman to fish" rather than "giving her a fish". So I never meant to tell you what to do. I imagined that you would download some Naples and Sorrento ferry schedules to get an idea of what the schedule might be like when you are using them.
I also never meant to imply that they cancel an entire day of ferry runs just because of rain. That's one reason the ferries have sealed, dark-tinted windows. It's more a matter of wave action, at the time of the ferry departure. I did imply that I would never plan to use the last ferry of the day, which is related to sunset.
I have not attempted to download a ferry schedule, but I don't see a lot of risk to planning on a 1PM ish ferry the day from Capri to Naples, preceding a night in Naples. Yes, there's some risk of getting wet while seeing Capri. But lots of tourists want to do their thing, rain or shine. You cannot make your Capri decision without knowing how many ferries a day there are likely to be, by multiple companies.
You could also say there's an emergency fallback of the shorter ferry ride back to Sorrento, and a pricey black car to Naples. (Just an idea, I haven't done this. But Capri is not the far side of the moon. Ischia, maybe.)
I'd comment that even before your most recent comments about "relaxing endpoint", I tried to convey the idea that Capri is beautiful, but that a comprehensive day is an exhausting amount of walking. Now that you have revealed that your DH has walking issues, everything is different. It's hard to see Naples, and impossible to "see" Capri without walking. Sitting by a nice pool in Capri is not much different that sitting by a nice pool at the Ambasciatori, or taking the elevator down to the private boardwalk Gulf swimming platform it has. This new fact changes everything, including a measure of the (Naples station) walk from the Frecciarossa to the Circumvesuviana, and from the train station to the hotel in Sorrento. Why pay 15 Euros for a Sorrento cab and decline to pay 115 Euros for a black car from Naples that day?
https://community.ricksteves.com/travel-forum/italy/changing-trains-in-naples
I do not agree that Naples is dangerous for daytime sightseeing by conventional tourists. It's true that northern Italy has starved the south for generations of infrastructure and business development. But Naples is no scarier than, say, Athens, where they also park on the sidewalk and drive in a scary way.
https://community.ricksteves.com/travel-forum/trip-reports/surviving-naples
After you evaluate (Google maps) the local walk from the Circumvesuviana, you might consider a past comment I have made on this board, that Oplontis might make a decent substitute for Pompeii for someone with limited mobility. Of course, I don't mean an academic substitute, I mean a swell ancient place experience for an hour or two.
Have you done some searches with the blue box top center yet, for additional thoughts?
https://search.ricksteves.com/?button=&date_range=2y&filter=Travel+Forum&query=capri+ferry&utf8=%E2%9C%93
Edit: I agree that the hard, rutted, un-even stone streets of Pompeii are a challenge, even for someone simply with weak ankles who can twist their ankle on good cobblestones. I don't think Herculaneum is a good substitute for Pompeii because it's a steep 15-minute walk from the Circumvesuviana station. Consider also Ostia Antica, easy daytrip from Rome.
If museums are not your thing, consider Caserta Palace, train from Naples. I have not seen them, but I would have liked to see The Cave of the Cumaean Sybill and "Virgil's" Tomb. Are you interested in the Naples path of "That Hamilton Woman"? Don't forget Naples pizza and baba au rhum. In fairness, there are plenty of Naples-certified ovens in the US now. But the flour is not the same, like in French Baguettes.