To answer both questions, I have the att version but no service except through Bluetooth to my note 5. However I would think you could turn on Samsung pay notifications in the gear app on your paired phone and get a Samsung pay notification on the watch through att service on the gear s3 watch without the phone paired. That's how I get mine. I have Samsung pay notifications turned on in both my note 5 and gear s3. When I make a purchase with Samsung pay on my watch I get a notification on my watch immediately or shortly after if I'm paired to my phone for data service. If my phone is not there, I get the notification when it's paired again. When I use my phone with Samsung pay, I get the same quick notification. Again only if my phone has data service at the time. But I do get the notification later when data service is reestablished.
Think of it like using your regular debit card. How do you know if it went through? It prints a receipt. Then you can check online to see if it went through. But you need data service to check. When you use Samsung pay via mst it us no different than using your physical debit card. Now if you use nfc, that's a different ball game because it has two way communication with the terminal and you get a visual indicator it accepted payment. This can be done without data.