You can make it up as you go along. The only limitations are trains with mandatory reservations.
Entering a train in the Railplanner-app doesn't book anything, it just gives you the ticket for the train. All seat reservations are separate from the Railplanner-app.
Yes, that's possible, as rvdborgt wrote. You can change, add and delete as many trains as you want. Thats what I did. Conductors may check if you wrote down the right train. Total freedom, except for the trains that need reservations. Especially with delays you might change plans, but make a screenshot of your original plan if you want to claim a refund.