You can add the pass to the app but there’s no need to. Stop the process before it asks for a passport/ID number, only do that right before travelling. There’s no need to have an active pass to book seats.

DB can book reservations in Germany and for their trains to neighbouring countries (although not to France as they’re mandatory and DB can’t handle that).

Yes when they’re optional, you can simply log the train and hop on!

Depending on where you’re travelling reservations might be mandatory. Some routes are more popular than others and need to be booked well in advance (Eurostar is one of them). Where are you heading ?

Have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm

The validity period will only start if you go through the activation process in the app and select a start date. You don’t need to do that in advance, the only requirement is that you do that before you board your first train.

Reservations optional = not mandatory. So yes, you can just jump on the train. Some seat reservations may still be recommended though.

Have a look here:

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

And for reservations here:

https://www.seat61.com/interrail-and-eurail-reservations.htm