In my opinion, the Swiss Travel Pass Flex is the worst option to choose. The Swiss Half Fare Card, including the complimentary Swiss Family Card, is in most cases the best deal.
Whether the Swiss Travel Pass offers are refundable depends on the provider from which you bought the ticket. I, personally, suggest buying it from SBB or Swiss Travel Center. You can find the refund policy on their respective product websites. For SBB, you can cancel it anytime before the validity start date.
Swiss Travel Pass – the All-in-one-Ticket for Switzerland | SBB
Swiss Travel Pass | Switzerland Tourism (myswitzerland.com)
You can find our English-speaking SBB Community here: Swiss Travel Pass | Switzerland Tourism (myswitzerland.com)