1. When the extension application is still pending, if you still stay in US, your current TN status is still valid (even the current TN is already expired )untill your extension application decision is made
2. When you leave US with expired TN, you immediately loss the status even your extension application is on pending. And you extension application become invalid also after your leaving
3 . You CANNOTuse the application notice to reenter the US
4. When you re-entered the US, you will need to go through the TN application process again, same as what you have done when you applied the first TN.
5. I believe mail-in application are much easier to pass as long as you meet the requirement. At the border, it also depends on mood of the POE officer at that day. Someone is not that rational
2. When you leave US with expired TN, you immediately loss the status even your extension application is on pending. And you extension application become invalid also after your leaving
3 . You CANNOTuse the application notice to reenter the US
4. When you re-entered the US, you will need to go through the TN application process again, same as what you have done when you applied the first TN.
5. I believe mail-in application are much easier to pass as long as you meet the requirement. At the border, it also depends on mood of the POE officer at that day. Someone is not that rational