First of all, but you shurely know it already: With the Friends Day Pass for Youth, all passengers must be under the age of 25.
You do not have to make a seat reservation for the Jungfrau Railways. Without seat reservation, it could be that you must wait longer.
Concerning your second question: The Friends Day Pass is only valid on the GA Travelcard routes, see here. You need an additional ticket from Grindelwald Terminal or Wengen to Jungfraujoch. There is *no* reduction, except all of you have the Half Fare Card. Without the Half Fare Card, GA Travelcard, etc. you must pay the full amount.
You can book the tickets for the route Grindelwald Terminal / Wengen to Jungfraujoch here: https://www.jungfrau.ch/en-gb/jungfraujoch-top-of-europe/buy-jungfraujoch-ticket/
The regular price is around 215 Swiss Francs per person.