Hey @jiutiao1001! 
If an Entrant is a winning Entrant, this is when we will confirm the eligibility as a Student Entrant.
I appreciate your patience in us getting back to your second question. Your video file should be encoded directly from Unreal only, using any methods in the documentation. We reserve the right to ask you to provide a screen recording of running the render from Movie Render Queue.
I hope this helps clear that up!