Trains will appear soon on Rail Planner, don't worry. It is not a problem if they're not there right now : you only need to log the train before boarding. On the day of travel search the connection -> trains will be there. This can be done 30 seconds before boarding.

Seat reservations are totally separate from Rail Planner app. Whenever asked you show both the app (QR-code, your ticket) and the PDF (seat reservation). You rarely need to show optional reservations.

It is best to use bahn.com or DB Navigator for planning. You can buy the seat reservations there too.

General info : https://www.seat61.com/how-to-use-an-interrail-pass.htm