WAAAAAAY too early-and no panick at all about this assumed ´full´ness. With trains ev hour or even more, you will find one with some seats even on the day-or before when you are in IT already
Hi,
do it like Sven suggest,
book the reservation via tickets.oebb.at after selecting departure and destination and the time please click on one way ticket and NOT only seat (I know strange you want only a seat reservation, but for Italy on ÖBB it works so) and than you get the trains, you have than click on adult at the top on the right and select as discount Interrail/Eurail Globalpass.
For Frecciarossa, -argento, -bianca and EC within Italy it costs 10€
IC 3€
EC to Switzerland and Germany via CH 11€ (2nd class)
EC to Austria and Germany via AT 10€ (2nd class) - only optional reservation (DB 4€/trip or ÖBB 3€/train, but 10€ fee for Italian part)
EC to Slovenia and Austria via SLO fee around 3€ - only optional reservation via ÖBB 3€/train
Trenitalia's app gives the train you want to take. But after research I see that there is apparently a change in the timetable from August 8th.
I will never use the interrail website to reserve seats. For Italy you can use the OEBB website, but it still shows the old departure times if you want to book, and no possibility to make a reservation. So I would contact OEBB to report this.