It is great to read that you plan to travel on Swiss public transport. Even for seniors, traveling by train in Switzerland is so easy, also thanks to modern trains providing low-level entry.
When looking up connections, the SBB Ticketshop offers from-prices, starting by default with the Supersaver tickets, if available. Connections for which a Supersaver ticket is available are marked with the % sign. In order to see the entire list of ticket options, you have to click (open) the connection. The standard point-to-point ticket is always available and will never sell out. You could even buy it at the station right before departure - the price is the same as online.
You are correct, when arriving by air, it is recommended to buy a standard point-to-point ticket rather than a Supersaver ticket which is only valid on the prechosen connection, and if you miss that train at Zürich Flughafen as the flight was late, it is no longer valid for a later connection. However, depending on the cost, I suggest you to look at the Saver Day Pass. it starts from CHF 52 (full fare, 2nd class) and is available 60 days out. The price increases the higher the demand. If your standard point-to-point is more costly than the Saver Day Pass, you are welcome purchase the latter; it is valid on any train for the day of validity yet is not refundable.
Furthermore, I suggest you to download the SBB Mobile app, it is brilliant for travel on public transport in Switzerland. With it, you can also buy tickets and it always offers you the best ticket for your journey. The Saver Day Pass is available there as well. And you can present the ticket right within the app.
And to finish, another personal travel advice, if you admit: when travelling down to Tirano, I suggest stopping at “Alp Grüm” station (google for it) for either lunch or a coffee. It is a train station that is only accessible by train and offers breathtaking views on the valley and the glacier. There is a restaurant in the station offering a terrace. And, while you watch the train leaving the station, you will feel yourself a bit left behind and abandoned, as the only way to get away is by the next train. Also, Pontresina is worth a stop for a stroll.
Let us know should you have more queries on purchasing tickets or using public transportation.
Have a good travel; kindly yours,