Actually I would rather call it weird when you need pass number. Seat reservations shouldnt require any information about which ticket used or about the owner. unfortunately some companies just want to randomly collect data.

Your ÖBB seat reservation is valid, but please note that you also need a supplement for those EC’s. I think its 10 euro and I think it can be paid on board (no clue if you also get it online).

I already paid the 10 euro per each train while doing all of the reservations so I think I’m done with that now

Bookings for Italy are much better made through the ÖBB website and you have saved both trouble and money by making the reservations there. Interrail charges a fee  of 2 EUR per ticket in addition to the fee for the reservation.

It DOES make some sense-or at least in the ways the railways themselves look at it: they do NOT want anyone to know they can just REServe and nothing else-this opens widely the doors to fraud and is in fact exactly the reason why several railways do not offer this anymore-or at least not via an easy way.

Imagine a noisy football club full of drunken louts to spend just a few € and claim the whole car?

you can only claim the seats you really use (by seating on it). no matter how many you reserved ans paid for reservation. but actually, not the best example, the drunken football louts probably get the whole car anyway….