Yes, no problem
I would think getting off early would be no issue, but I’m asking about getting ON at a different stop than my ticket says.
If you get on the train on a stop that is subsequent at the stop your ticket says, you don't use the train for some kilometers you've paid for. Your choice.
Ok understood. We booked the tickets before knowing our full itinerary. We might be closer to ostiense
There is no checking of tickets as you get on a train. The conductor may come around while the train is moving to check tickets, and you will have a valid ticket when it is checked,