To find availability I used tickets.oebb.at (train's co-operator) : add Interrail as a discount and select one-way tickets. 13.70€ per seat
Did you book the ICE between Utrecht and Cologne/Frankfurt ? 3€ through ÖBB. It is very popular and has mandatory reservations in summer due to overcrowding. There are regional trains across the border if needed too.
Mmmh not sure about that route from Rail Planner as it is indeed the same train coming from Stuttgart, Augsburg and Munich... The app isn't really reliable, best use DB Navigator app or bahn.com.
As alternatives :
- seats on Saturday or Monday
- sleepers (51€) on Saturday between Zurich and Ljubljana
- seats (12.90€) on Sat/Sun/Mon between Zurich and Graz. Couchettes (31€) on Saturday or Monday. From there a new pass day the next morning or a regular ticket
Do not hesitate for questions.