Seat reservations are totally separate from the app. They won't appear there, even those made on interrail.eu wouldn't.

Whenever asked you show both the app (pass) and the PDF (seat reservation).

It is recommended not to activate your pass in advance. In case you wake up ill, trains are cancelled, etc. you could lose a pass day.

There is the big European-wide timetable change on 10th December. Most timetables have been published on the companies websites but not yet on Rail Planner. You don't have to add the train manually now, it will probably be uploaded soon. Remember : you only need to add the train seconds before boarding.

The best places to search for timetables is the Deutsche Bahn website at bahn.com/en. It's likely to be more up to date than the Eurail app.

You can then add the journeys to the app later, there's no hurry. You can add them manually if you need.

I want to buy the night train tickets in advance because it might get full or more expensive. Swedish tickets get more expensive the more full a train is, but maybe that doesn’t happen with only seat reservations.

The reservation is separate from the ticket.

Seat/ sleeper reservations won't get any more expensive (except for French TGVs, which can go up from €10 to €20), but they can sell out. You should definitely buy the reservations as soon as you can.

The app is just a record of the trains you took, for Eurail to decide how much money to give to each railway company. If you buy a paper railpass, it's literally just a piece of paper you write your trains in, no more than that.

The price for seat reservations is fixed.