Correct, you can run a B division with the remaining 50% if you want and that is fine also.
But if you include more than 50%, you can still include the players in the final ranking, it's just the games played don't count toward TGP.
Nope, games played
As an example, each session, the league could be playing 8 games with indirect qualifying. So if you play a full 4 weeks in an attempt to actually qualify, that would be 32 games total.
You could have a casual player that shows up for 2 of the 4 sessions and only plays 3 games each night, meaning if we did it by session the player would be able to be included, even though they only played 6 of the 16 games that the other people included played.