I must be doing something wrong. On the occasions that I have to use MST rather than NFC, the transaction is successful, however, it does not register in Samsung Pay. I've tried three separate transactions on three separate days. Nothing. Any clues?
According to Samsung's FAQ you can trade in a financed phone, but you're still responsible for making the payments to the carrier.
Still should inquire with your carrier.
It's dependent on the app itself. If you go to App Info > Notifications > on lock screen for Facebook messenger, selecting to hide content will display the contact name but no message. Textra also has similar settings, in-app.