Hi Josh, and welcome to MySwissAlps,
>> We have booked rail tickets (no seat reservations yet) from St. Moritz to Tirano for our trip on July 20. The tickets we booked were for the 9:39 to 12:00 BEX labeled train through SBB site.
Normally tickets do not have a specific departure time on them, just a date. Is this the case with the tickets you bought?
>> Knowing we have booked this leg specifically, do we have the option of hopping on an earlier regional train instead with those same tickets?
If you don't have a reservation, you have bought tickets for a route and a day, not made a booking for a particular train (unless you have bought Supersaver tickets). Have another look at your tickets to confirm.
>> I have heard that a ticket purchased is good for the whole day on either regional or BEX, I just wanted to make absolutely sure.
It is only good on the Bernina Express if you ALSO have a reservation, because reservations are mandatory on that train.
As of today, there are still 25 seats available in 2nd class on the 9:39 Bernina Express departure from St Moritz on 20 Jul 2018, including 10 window seats. If they are all booked out by 20 July, taking that train will not be an option for you.
>> Just in case, does the train from St. Moritz to Tirano start with the second class cars?
Do you mean that the second class cars will be directly behind the locomotive? Not necessarily. They could be there, or at the other end.
On a regional train, you will get no warning of the approach of the Brusio circular viaduct. As you approach it, it is on the right between Brusio and Campascio, but not all trains stop at those stations, so keep an eye out for it once you have left Miralago (at the far end of Lago di Poschiavo/Lake Poschiavo).
>> In general, will the 8:48-11 regional train be rather empty?
Possibly, but in July that is anyone's guess. The SBB timetable does not seem to show occupancy status for trains on that line.
>> Finally, if booking a "super-saver" ticket, does that mean you have to take that specific train?