First. YES they will take the EuroPass. don't worry about that.
Second the Panoramic train, MAY or MAY NOT take the Europass.
I do not where you live, check with the Swiss Bureau of Tourism. They will have the answer, or sometime even the consulats.
Geneva-Bern, you will stop at Lausanne, as fas as I remember.
Second: From Montreux go to Vevey (have some cold white wine at a restaurant) and from there go to Bern. Not much to see in Lausanne in one hour.
Also you may not have enough time for your connection in Zweisimmen (5 min.) This is my opinion.
Do not forget : TRAINS LEAVE ON TIME (arriving even one min. late, you will miss it)
Yes, it is quite ambitious.
Have a nice trip anyway