Hello JTfromAK
The combination of the Swiss Half Fare Card and the Bernese Oberland Pass indeed offers great value. You may purchase the Swiss Half Fare Card in advance; however, I would not do so for the Bernese Oberland Pass. There is no advantage in purchasing it in advance, as you never know what will happen until August 2024. In case you cannot travel, you will have to go through the hassle of claiming a refund. I would suggest purchasing the Bernese Oberland Pass while you are in our country; you can well purchase it while in Switzerland, at any SBB travel centre.
The Bernese Oberland Pass is a private tourist offer; it is sold by SBB, acting as a reseller, but cannot be integrated into the SBB Mobile app.
The Luzern-Interlaken Express is operated by Zentralbahn, not SBB. You can reserve the seats using their seat reservation tool, available here:
https://store.luzern.com/luzern_en/specials/daytrips/zentralbahn-int-lu?
I suggest reserving a seat on the right-hand side in travel direction, for better views.
Another question: do you plan to stay in Zurich? I would rather suggest staying in Lucerne, which is more Swiss, has more ambient temperatures in August (due to the close mountains), and offers more mountain excursions and lake trips.
Please let us know if you have got further queries.
Kind regards,
Roland