once you submit the stuff through the app, the QR code is valid for 4 hours. No doing it in the lounge prior to departure unless it’s a very short flight.

The QR remains in the app under receipts for 4 hours.

You do not use the kiosk at all. You simply skip the kiosk which helps you move forward in the line and go straight to the MPC line. The officer sometimes scans the QR code, other times they don’t.

You can do multiple receipts/people at once. You just swipe left and right to display the next QR page.