Bernina route questions

Hi,

I'm planning to do the Bernina route on normal trains, early morning. rail.ch shows me a timetable where I'll be able to go till Poschiavo, but then take the bus onwards to Tirano.

1) If I buy a standard ticket from Chur to Poschiavo, it will be valid for all trains on the route (i.e, I can hop on and off trains?)
2) How do I pay for the bus(if I decide to stick to the suggested schedule), at the point of departure?
3) Is the bus route from Poschiavo to Tirano the same as the train route, or will I be missing a lot?
4) Can the bus wndows be rolled down, so I could take pictures?

Thanks so very much

Thanks Arno