Hello Kabagoaf89
October (until 20 Oct) is a great month for travelling in Switzerland. The mountains stand out more in September and October, thanks to the lack of humidity in the air.
Please kindly note, the correct term for the Swiss Half Fare Pass is Swiss Half Fare Card.
The Swiss Half Fare Card has to be purchased latest on the day you want to travel with reduced fare tickets. There is no need to purchase the Swiss Half Fare Card now for October. You can purchase reduced tickets now, and the Swiss Half Fare Card anytime later. There might be unforeseen changes to your travel plans; spending the money that far in advance doesn’t seem reasonable to me. The Swiss Half Fare Card does not sell out.
The Saver Day Pass offers you can find on this website (press the Purchase online button):
https://sbb-tickets.cc/en/tickets-offers/tickets/day-passes/saver-day-pass.html
The Bernina Express has coaches that need a seat reservation, yet there are also coaches as part of the same train, for which you don’t and which offer free-seating. For the latter, you do not have a guarantee of finding a free seat, as these coaches are used by the locals for commuting, too. If you want to have a seat, do reserve your seats. The right-hand side in travel direction offers better views.
Does that make sense to you? Let us know if you need more information.
Also, since the Bernina Express is operated by Rhaetian Railways, not SBB, you might want to ask the RhB rail service for Bernina Express seat reservation related questions:
https://www.rhb.ch/en/service-souvenirs/contact/railservice
Kind regards,
Roland
Ps. One last note: you did not say how you will travel from Zurich to Interlaken. I suggest travelling via Lucerne and spending some time there (if not even spending a night). Don’t waste too much time in Zurich, our country has more to offer than Zurich.