1 - It does run. For the panoramic coaches, you do need a mandatory reservation (CHF 32 per person). Reservation-free (normal) carriages at the front of the train (24 places 1st class, 76 places 2nd class).

2 - Yes, that’s possible. Every hour from 6.58 until (to get to Milano the same day) 15.58 (change of trains needed; 15.58 by bus from Poschiavo to Tirano, but it’s not a good idea to do this trip in darkness anyway). 

3 - There is (every hour) at x.08. It takes 2 hours and 32 minutes. Last train at 20.08. You need to change station buildings at Tirano but they are nearby (takes less than 5 minutes).

Maybe to consider - It’s possible to have breakfast or lunch in a nostalgic restaurant carriage on the way to St. Moritz when leaving Chur at 08.58 or 11.58 (should be reserved). Would mean to arrive in Milano Centrale at 15.40 or 19.40.

Thank you Hektor very much.