I took the Bernina Express last June from Tirano to Potresina...(right outside of St. Moritz).
I bought a 1st class ticket ahead of time. Larger seat, etc., but nothing special. In the carriage, there were only 4 other people (family).
I agree....the un-openable windows do hurt the possible photography. The scenery is absolutely beautiful. While there in June, it snowed! Beautiful lakes, glaciers, etc.
The train did stop a little at the Alp Grüm stop. People got off for about 5 minutes and took photos of the glacier.
With the Bernina train...you can't hop on/hop off. But with the regional trains, I think you can do that.
It depends on how long you want to spend. There are a ton of trails. I would love to go back and do some hiking and biking.
I went to Pontresina and spent one night (got an AirBNB room). In June...it was super quiet and I didn't get a chance to really do a lot. The town is very small. St. Moritz would be a good choice to see and take some of the lifts up to the peaks. Also, Diavolezza lift looked like a good thing to do!
Day trip...from Como, would be super long. The others are correct. I rode trains from Milan to Tirano and it was a long day of travel.
Overall, it was a nice experience, relaxing, especially after 12 days in Italy doing city tours.
I also bought a $25 USD hamburger, fries, and coke in Pontresina. Oh my....Switzerland, you're expensive!