Hi
To add journey´s to your pass you need no Internetconnection :)
the app need it just every 3rd day :)
depending on the trains you take you maybe need reservation these can be booked online or at the ticket office

Hi
um die Fahrten deinem Mobilepass hinzu zufügen brauchst du keine Internetverbindung :)
Jenachdem welche Züge du nutzt brauchst du evtl. noch zusätzlich Reservierungen
Diese kannst du Online oder am Fahrkartenschalter kaufen (beachte die Warteschlangen in Paris sind relativ lang)

I'm not sure what your question is.

When you buy an Interrail, then that is your ticket. You can use it for whatever journeys you want (with restrictions in your country of residence) and you don't have to indicate anywhere in advance what journeys you want to make. Only before you enter a train, you must fill in your trip report (paper pass) or add a journey to your trip and your pass (mobile pass).

If you want to make a reservation for a train, which is sometimes required, especially for long-distance trains in Spain, France and Italy, then you can do that at a ticket office or often also online.

You can check in a planner (including the DB planner) if reservation is required. As to the journeys you mention:

1. Paris - Rouen: some regional trains (depending on the destination) sometimes require a reservation (Friday-Sunday and bank holidays). This is a new invention of French railways (or the region). Reservations should be available at the SNCF ticket machines or at the ticket counter for little money. The rail panner app and DB planner say reservation is mandatory for all trains to Dieppe but that seems to be incorrect, since the SNCF website says it's optional.
2. Rouen - Dieppe: some trains are without reservation, others (the ones with fewer stops) have optional reservation, as mentioned above.

If you have any specific reservation questions, then it's best to mention the specific train(s) you want to use, with date and departure time(s).

@1: update: nearly ALL trains now-and certainly the faster, are labelled NoMAd and these MUST be reserved from Paris or to Paris-not for other trips (f.e. Rouen-le Havre. It is unclear if this is free or also cost money. This is quite recent and was also mentioned here recently as update/new info. There are gates  at the Paris statin St.Lazare for the platforms of these trains, to be opened iwth the barcode on the REServaton

The direct trains to Dieppe are labeled "KRONO” in the PDF timetable. The SNCF planner says "NOMAD” indeed. Neither one mentions mandatory reservation. The only reference I can find is the page I mentioned above, which says mandatory reservation only exists in some trains on some days.

In addition, the gates do not determine if you have a valid ticket or not.

Edit: NOMAD is the name of the entire TER network in the Normandy region. Only some trains have mandatory reservations.