Are you that set on getting the ZHT-ZRH train segment onto the itinerary? Concerned about being protected in case if delays/ misconnect?

For starters, in my experience with call centers in Philippines and such, they will have a different POS (point of sale) than M&M website (Germany) and those agents couldn't even book the LH train segments that website showed either. They said they don't have access.

If it was me, I'd just book ZRH-FRA-IAD ... trip that is available online and go with that and buy separate train tickets from Geneva and buffer my connection time to have at least 1 subsequent train with which I could still make the connection. A risk remains though in such cases. An overnight stop in ZRH would be another option, but more cost for hotel.