There is also an obligatory 9 CHF seat reservation for Bernina Express Chur - Tirano - Chur (it's not clear to me, is it for one-way, or for return trip?).
One way, and you can chose your seat right away when purchasing seat reservation online.
can I use TCV ticket for this route, and for Bernina Express specificaly?
Yes, you can. That is exactly the point - sometimes TCV (or as they called these day SCIC-NRT tickets) bought abroad are cheaper than domestic or international tickets on certain connections. The only thing is that you can't purchase such ticket in Switzerland or Italy, but need to bring it with you from Czech Republic, Slovakia or even Hungary and Serbia (if cashiers skillful enough and have full TCV tariff books)