Hello Jerryfai
I think that it is better to buy tickets with your Swiss Half Fare Card from Geneva Airport to Interlaken Ost for the first and last days of your trip and don’t include these days in the Berner Oberland Pass.
Don’t buy a Supersaver Ticket between Geneva Airport and Interlaken, if your flight is late this ticket will be no more valid.
For the last day, you can buy a Supersaver Ticket, but you must take the selected trains and such tickets are not refundable.
With a Swiss Half Fare Card, you can buy the Berner Oberland Pass at a reduced price.
Kind Regards
Danhiel