So how/when are the animations that go to the national competition going to be decided?
Whenever Autodesk is done deciding.
which is never xD
All animations are entered in the Championship Event, whether you win a Regional or not.
In the past Autodesk picked the top 5 about a week before the Championship & invited the teams to a special (free) luncheon. The overall winner is announced at the awards ceremony Friday.
They do the same thing for the top 5 Autodesk Inventor entries.