Jun 16, 2016 - 1:51 PM
1./2. You will travel for free to Grindelwald, and from Wengen on the way back. The other legs are 25% discounted. You can only buy this discounted ticket locally, at any staffed train station in Switzerland.
3. Reservations are possible from Kleine Scheidegg to Jungfraujoch, please see www.jungfrau.ch.
While I would normally recommend to just buy your ticket on the day and not book seats, August 1 is the National Holiday, so it could be busy if the weather is good. You may want to buy your ticket, and optionally the reservation, one day ahead of time. That's just to be sure they won't be sold out. Please see here.
All of the above assumes you have a Swiss Travel Pass, not a Swiss Pass.