What you have to understand is that tickets are for transportation, they are not for trains. In fact, in Switzerland there is no such thing as a train ticket.
If you want to travel Geneva - Somewhere via the Golden Pass express you just buy a ticket Geneva - Somewhere, and it allows you to use any service on the route you bought, on the day your ticket is valid.
So a ticket does not guarantee you a seat. If more people board a train then there are seats then yes, some people will have to stand. This is however rather rare on Swiss trains. Usually there is enough place for anyone, and you do not need a seat reservation.
Trains are mass transit, used by people to go about their everyday business. Having to buy tickets in advance is not compatible with that. Imagine you would have to buy tickets in advance to use the NY Subway.... Would New Yorkers accept that? Well Swiss would not accept not being able to buy tickets whenever they want, and use trains whenever they want.
So you can safely buy tickets at the last moment. Just wait till you are in Geneva, and buy the 1/2 card there. Then just buy whatever tickets you need as you go...