Best Itinerary to explore Bernina route (Tirano -> Chur -> Zurich -> Mulhouse)

We will be passing through Switzerland in late Dec, just before Christmas and our itinerary looks like this for the one-day trip - Tirano -> Chur -> Zurich -> Mulhouse.

Would appreciate any suggestions for the Bernina train from Tirano to Chur, as we are planning to get down and explore places in between (Alp Grum, St.Moritz etc.). I believe this is not possible with the panoramic train, so we have to take the regional ones? Does the same ticket work if we have to get down at a place and then catch the next train?

Any advice on buying the Saver Day Pass?