Assuming your outbound and inbound are earning equally at 10,500 miles each, when you are on your return journey you will only have 28,000 miles in your account and won't be able to upgrade. After you land back at "home", you'll get credited the other 10,500 for the reasons given above.

If your outbound is earning more than your inbound, then you should be OK and yes, you can do this at the airport. The lounge might be the best bet.