Could Dumais play for another team/division in juniors, wasn't that one of the complaints that a lot of his points have come against 'easier' teams because of the division stuff? I do remember reading somewhere that the Canadian junior teams are essentially fighting for themselves and are a league in their own right so maybe they don't like the idea of trading players, but playing in a tougher division would sound like an easy solution otherwise...
I am not 100% sure where you are going with your question but if I understand correctly I think the suggestion that one Junior league is tougher or easier than the others is a bunch of hooey. There are three top level Canadian Junior leagues: QMJHL, OHL and WHL. They are set geographically and I believe what province (or state) a player is from dictates which league they will play in. Dumais is from Montreal, he was always going to play in The Q. Trades happen all the time within the leagues but I don't recall there being trades from league to league.
That said, I think decades ago The Q had a reputation for being high scoring with no defense. I think over the last couple decades that has proven to be false. No league is necessarily better than any of the others. Last year was an exceptional year for the WHL in the first round (six players drafted) but go back another year and the WHL had 4 first rounders, The Q had 4 first rounders and the OHL had just 1 first rounder.
Perhaps this is just my opinion but Dumais jumping to the OHL or WHL wouldn't necessarily accelerate his development any more than staying in The Q. Playing with and against 16-20 year olds is still playing with and against 16-20 year olds. I think what he needs is to play against more experienced - more physically developed players which he cannot do staying in any Canadian Juniors league. But it doesn't matter, the CBJ might not wanna send him overseas, he might not wanna play overseas and his Q team might not be willing to let him play overseas. So the point appears to be moot based on what someone said above (once a player plays in the Canadian Juniors they are apparently owned by that organization until they turn twenty or they are good enough for the NHL or their Junior team doesn't want them anymore). Seems too restrictive to me.