While I think your proposed CP requirements in Modern are about 50% too low, I think you've put together a great argument for the league locks portion.

RE Part 2: Decoupling from tournaments

I wouldn't say the previously implemented restrictions were a failed measure, but they do still have their flaws. I agree with points 1 and 2, however, it gets a little murky around points 3 and 4. Yes, cards owned by Champ players are limited to the league of the tournament and yes, the meta is different in lower leagues, but a quick look at tournaments results down there will show you that doesn't seem to be much of a problem for them.

First, you can't ignore the significant card advantage they bring with their collections. Yes, other players could rent those cards to lessen the advantage, but in a lot of cases, that's just not financially reasonable if you're hoping to come out ahead.

Second, even though the meta is different, that doesn't seem to make much of a difference. We constantly see Diamond and Champ players dominating Bronze and Silver tournaments. Some of it is due to the card advantage that comes with having a larger collection and some of it is skill.

I can't speak much on point 5 because I don't pay attention to the number of entries in Novice, Gold and Diamond tournaments, but I can say that both Bronze and Silver tournaments still manage a pretty good amount of players.

I've always been a fan of the "one up, one down" idea for tournaments and I think that could still work well in conjunction with your league lock proposal, but there would still be a couple things that would need to be addressed. One up, one down would function exactly as it sounds. you can enter tournaments one league up or one league down from your chosen league. Being that Diamond and Champ players wouldn't be allowed to enter Bronze or Silver tournaments anymore, the number of Diamond tournaments would need to be increased, and even though the card levels are the same between Diamond and Champ, I also think some Champ tournaments created.