For non Reservation compulsory there is no problem as you can take as many trains as you like on a travelday just take the next one that brings you to your destination  If the train reservation compulsory is the bestway to exchange the reservations for the new train at the ticket office of the station where you change.

I can only use as many Trains as I have listed in my mobile pass, or?

Yes, which means you also need to update your schedule in your rail planner app. This way everything is still filled in correctly. It applies to both trains with and without a reservation. If you change your reservation at a ticket desk, it's usually free of charge if it's beyond your fault.

And in case you're short on time, you can also just go to the train conductor and ask if there are free places available (if you had a reservation for an earlier train) It happened to me once when I was in Paris, there was a fire and the queue for the ticket offices were at least an hour, so I asked the train conductor of the Thalys for free places.

Can I update my schedule in the rail planner app during a travel day?

ofcourse :)

Oh, that's great. So if I miss a train, I go to the railplaner app, choose an other train and go with this train

Correct thats the advantage of Interrail and Eurail that you usually doesn´t have a fixed shedule and can change when ever you like your trip. (Reservations make it a bit difficult)