Eurail doesn’t work in routes. It works by train companies. All trenitalia trains are included,  So no, Naples isn’t the furthest south. You can get to Sicily by train to catch a ferry to Malta.

thank you - just can't work out how to use and book.

when shall I book it if I'm going last week in may?

You can just work out what exactly? How to use the Interrail/Eurail pass and how to use it when wanting to board a train?

But before that all: You’re waaaaaaaaaaaay to early for next year in May. The new timetables for next year are not even known yet. On top of that, Italy is always on the late side with it’s schedules (usually known 1 or 2 months in advance).

To plan, you just take a map, look where the railways can take you and what you would like to visit. Use a reliable planner like www.bahn.com (it holds most rail timetables of all of Europe) with a random date somewhere next week to see how long routes will take, what frequency of trains you can expect and where it might be convenient for you to stop. It can be helpful if you want to book accomodation already.

Timetables usually don’t change that much over time.

Other than that, nothing to do but wait until February or March ‘24 before reserving seats on long distance trains or night trains.

thank you - that's a big help