You are correct that its very difficult to get decent schedules in advance. There are several different ferry companies, there are jet boats, hydrofoils, and larger ferries running different routes so it gets even more complicated. I believe there are 2 companies running between Naples and Capri (jet boats take 50 min and ferries 90 min). There is usually a service about every hour on the route, but it does vary depending on time of day. There is also pretty frequent service between Capri and Sorrento (about 20 min ride). But, the service to and from Positano is much less frequent (only a few times a day). As to Pompeii, the Circumvesuviana Line runs trains about every 30 min between Naples and Pompeii which have a stop at "Pompeii Scavi" (do not get off at the "Pompeii" station). Trying to do your itinerary with a boat to and from Positano is probably nearly impossible in a port day (because of the frequency of service to Positano). Also keep in mind that you need at least 2 hours (bare minimum) just to get from Marina Grande up to Capri, get a quick glance at Capri Town, than back down to Marina Grande (where you get the ferries). Personally, we would never going to Capri for less than 4 hours. Pompeii also takes hours (we have been there 3 times and still have not seen the entire place) to fully enjoy although its possible to see the highlights in about 2 hours.
As to schedules, I suggest you simply keep your flexibility. When you get to Naples, simply walk off the pier and than turn left where you will see the ferry terminal (a short 10 min walk from the ship). At the ferry terminal, you will see a group of ticket booths (representing several companies) and they will all have their daily schedules posted. There is also an information booth. At this terminal you can quickly assess all of your options for that day and act appropriately. You really do not need reservations on most of the boats (although its possible to buy your tickets in advance that day). However, if you do ultimately plan on taking a late afternoon jetboat from Capri back to Naples, and there are a few ships in port, its smart to buy your return ticket early in the day or as soon as you arrive on Capri...since the later afternoon jetboats (usually around 3:30) could be very crowded with cruise ship passengers (even many of the tours use these same boats).
On a personal note (take this advice or leave it) your post makes me think you are trying to cram too much into a single day. In the end, you can spend your day running from place to place without seeing anything. If you want to do the Amalfi Coast (Positano) this should normally be a full day with visits to Positano, Amalfi and Ravello (generally done with a private/car driver...but can be done with public transit). Capri and Pompeii are also possible in a day (although you will still be rushed in both places). With a private driver/guide it is possible to do Pompeii and the Amalfi Coast in a day (somewhat rushed), but this would be a stretch with public transit.