Bernina Express,, seat reservations required for scenic coaches?

Hello mate

I hold an 2nd class Eurail Pass and currently planning to travel on Bernina Express from St. Moritz to Tirano and Tirano to St Moritz / Chur

1. Is it possible to use the pass for these sectors?
2. I searched on internet that there are some open scenic coaches. Is seat reservations required for these coaches? Do they have a specific departure time?
3. I noted that some normal coaches on the Bernina Express train do not require a seat reservation, is this really the case? Do you recommend to make a seat reservation?

Thanks all for your help!