Hello Llamauqueaf66
The Centovallina, as the locals name it, is a very scenic and entertaining train line.
Saver Day Passes do grant unlimited travel during its validity on any public transport service in Switzerland (except mountain trains). So yes, it is valid for the Locarno to Domodossola route as well. In addition to your Saver Day Pass, you will be subject to a payment of CHF/EUR 1.50 per person when using the panoramic train, to be paid at the ticket window in Locarno or with the conductor.
As the Saver Day Pass offers travel at will, it can be used for your return as well. In fact, you could travel multiple times forth and back on that route 🙂
Please kindly note that the Saver Day Pass requires your passport or ID (no copies, no photos) upon ticket check. You will have to have it with you anyway as you will cross the border.
Kind regards,
Roland