You are right, about 50% of players entering the QMJHL come from the M18AAA or the M17AAA league.
The other 50% includes players from other leagues, respectively, other leagues from Quebec, the Atlantic provinces, the United States, Europe and a few from elsewhere.
About fifteen players come from "other leagues in Quebec" and this comes mainly from the M18D1.
What I mean, and you can look for "old guard" thinkers in some previous threads, is that not only M18AAA players will be on the CSR list... and some of them although not first rounders (I've already given some examples), might surprise us!