Aug 6, 2012 - 11:38 PM
We had travelled recently with a Swiss Rail Pass, which was called family flexi saver pass. That allowed 5 days of unlimited travel within Switzerland. Looking at your pass I could not tell if it is of the same type or not. Anyhow, I will try to answer some of your questions below :
In Basel you can go to the enquiry counter, which is towards on end of the terminal bldg. They will stamp your card there. In my opinion, 5 minutes is too short since there may be people in front of you in the queue. But as you have noted, there are many trains from Basel to Interlaken, you can check in www.bahn.de or sbb.ch.
We went to Jungfrau also and it is better to have flexibility for the date, since if the weather is not good, there is no point going there. We had to pay a reduced fare for that with our Swiss pass. From Interlaken you first come to Grindelwald and then change again at Kleine Scheidegg - you can get all this info on the net.
Finally, I would say that if you like natural beauty, try to stay overnight in Grindelwald as opposed to Interlaken. We stayed for 4 nights in Grindelwald and you fell within the lap of nature. Interlaken in only 30 mins. away by train but it is much more crowded.
Enjoy your trip.