The seats in the night train Amsterdam - Zürich run as a separate IC (although they're in the same train). You can book them via ÖBB if you choose "Seat only”. You'll see both the NJ (with couchettes and sleepers) and the IC (seats) with departure time at 20:30. Choose the IC; seat reservations (optional) are 3€.

As to the carer pass: the customer service should handle that but they're currently very busy and seem to take weeks for non-urgent requests. When do you start your trip and did you mention that to customer service?

Oh amazing thank you thank you! Yes I can see how to reserve now, brilliant. But now I'm in a real quandary, we don't go until July but I wanted to make sure we had all our reservations early because I have some extra needs so need to make sure everything is in place. But without the carer pass can I still book the seat reservations for all of us (including my carer) just on the hope they get back to me and issue an extra pass?! I kinda just wanted all the passes issued and in the app before I started reserving seats!

I'm surprised at the lack of a way to actually contact interrail, there seems to be no phone numbers or anything, how would a passenger who was blind supposed to manage this system? Seems incredibly difficult!   

You can book these reservations without having a pass.

There is no public phone number indeed. Normally, customer service does indeed respond after a few days but since April, it seems they've been inundated with requests.