REServ and trip administration are completely separate-but it is indeed mandatory to register all trips made-and in the app, as otherwise you do not have a valid ticket for that trip.

How the 2 communicate-or not-is more a case of your own ingenuity to let it happen.

As you can read in about any post re REServ the experienced pass-users avoid the app for REServ amap-and do it via other ways.

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

Regarding the reservations, the advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

Thanks very much for the links, now I understand a lot more. The last time I went Interrailing was in 1979 so things have changed a bit since then :)