Perche

I've never tried to do it, but I don't think so. If you take a regional train you have to stamp it or get it validated before you board. Once validated, it is good for four hours. So, you can't stay overnight in Pavia, then get on another train and continue the next day on the same ticket. Mobile online tickets are pre-validated, and are also only good for four hours. There are only regional trains to Pavia from Milan.

The travel from Pavia to La Spezia is a longer ride, and you'll be on a freccia or intercity. These will be tickets for an assigned train on a specific date so if you get off, and get on a different train on a different day with the same ticket, the conductor will consider you to be ticketless.

I'm not aware of on and off privileges, except that you can change to a train one hour later or earlier than what your scheduled one is, or to the next available train if you pay a fee of 8 euros, or somehow arrange for the conductor to give you a voucher or sticker for another train. The only way I know of to get on and off at different cities at will is with a weekly regional travel card, and that gets expensive going from one region to the next.

I could be wrong, but I just don't think you are going to be able to get on and off different trains on different days with the same ticket without it getting pretty complicated.