Hello Rob_e
Supersaver tickets are tied to a specific connection that was chosen during ticket purchase. The conductor who accepted the Supersaver tickets on another train than the one scheduled was doing a favour, but this is not the way it should be and actually not according to the conditions; this courtesy apparently left a wrong assumption and caused confusion at your end when the other conductor did not accept the tickets on the way back. I am sorry for this.
It is true, Supersaver tickets do kill any flexibility, yet, on the other hand, they are also very inexpensive. We do not recommend Supersaver tickets when arriving by air, as they are likely lost in case your flight is late.
That being said, the Saver Day Pass, however, allows you to use any connection. It is a ticket that you can use at will, and I would have had suggested this ticket had you asked me.
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
Save your passes as soon as possible, as the price is increasing the nearer the travel date. Also, as all e-tickets, passengers using a Saver Day Pass must show their physical passports upon ticket inspection.
For the way back from Zermatt to Geneva, however, you could actually purchase Supersaver tickets as you are certainly able to meet the departure time as foreseen by the ticket’s itinerary.
Please let us know if you are in need of further assistance.
Best regards,
Roland