Thanks for the detailed answer.
I’m a bit concerned about “old” sessions, though: if you check the top 20 players of the ladder, usually there are only a few submissions from the last week(s), all the others are several months old. Those players will probably not resubmit their code, because it’s already good enough 
and resubmitting causes an immediate drop in the ranking (as discussed here and here). But that means you will rarely fight the champions… and that means probably less chance for new submissions to climb to the top and less chance to challange the “champions”.
Dueling Grounds, for example:
Also, what is considered “old”?