>> However, I was not able to locate a screen where I could purchase seat reservations for bus 171 from Thusis to Bellinzona. I was able to locate a table on the www. postauto website directed to travel between Thusis and Bellinzona, but it does not appear to be directed to bus 171. (see attached)
Route 171 actually starts in Chur, and is therefore called the Chur–Bellinzona Route. It stops at Thusis en route. The route you found, Route 541 starts in Thusis, but actually follows the same route from there to Bellinzona as Route 171. The stopping patterns and travel time probably differ. You might get a better seat if you take route 541 though! Route 171 will already have passengers on board from Chur when it gets to Thusis. Reservations guarantee a seat, but do not specify a particular seat number.
Here is a link to the page for Route 171:
www.postauto.ch/en/lei sure/chur%E2%80%93bell inzona-route
About halfway down the page there is a paragraph heading "Reservations", with a link to reserve online.
When you follow that link, you get to a page with information about reservations, with a yellow box at the top of the page with the text "Chur–S. Bernardino–Bellinzona". Click on that link, which will take you to a form where you can fill out the From-To details together with date and time.
The "From" field will already be showing "Chur, Postautostation", as that is where the route commences, but you can choose "Thusis, Bahnhof" from the drop-down list. ("Bahnhof" means railway station in German. The railway station at Bellinzona is called "stazione", because Bellinzona is in the Italian-speaking area.)
After filling this out, click on "Find connections" and choose your preferred service. You can then fill in the rest of the details to complete the reservation. (See attached screen capture.) The reservation will be emailed to you.
By the way, there is an audio guide for this route. You can download the audio in advance and listen to it before you go as well as listening en route. It's accessed in the PostBus app, which you can download via this link:
I hope you enjoy the trip!