Jul 13, 2015 - 8:23 AM
If you're in the US you can normally book seats through www.raileurope.com/us/ rail/point_to_point/tr iprequest.htm, but indeed it does not seem to work right now. The link you found can be used as well.
You can buy a second class pass through www.raileurope.com/rai l-tickets-passes/swiss-pass/index.html (see here for all retailers). It does not matter that you will purchase your seat reservations through another website. The pass and the reservation are not linked in any way. You just need to have both when boarding.
An upgrade to 1st class can be purchased at any staffed station in Switzerland, e.g. Montreux. Just show them your Swiss Travel Pass, and they will sell you the right upgrade.
I'm not familiar with the current rolling stock between Zweisimmen and Interlaken, but as far as I know the views are good in both 1st and 2nd class. That even applies to Montreux - Zweisimmen, apart from the 1st class VIP seats in the front of some of the trains. They offer a very different view. Of course 1st class is more spacious and comfortable.