Well I feel that Fox example could be applied to Marth, but not to MK. Melee Fox has counters and is doomed to remain that 'attractive pocket secondary'. Almost everybody in France has a pocket Marth. Dany, Okals and Leon are the only ones that main him iirc. This is not really the case for MK (Blubo, Myollnir, Neath (and I but that doesn't matter, I suck at brawl) all switched to MK maining)
Melee Fox actually doesn't have counters (if so, I'd be curious to hear them). On paper he's still the best character with at worst some even matchups and I feel you underrate him as a solo main, because he gets consistent placings that way too (he placed 3rd, 4th, 2x 9th and 13th on APEX if you look at top 16, despite two of the best Fox mains not attending...). The one major issue he has is that it is hard to consistently tap into his potential and even bad characters can hurt him a lot for mistakes due to Melee's mechanics, making it often a better idea to use him for specific matchups instead of all the way through. The best Fox mains are overall consistent though and if Armada is to be dethroned, it's likely to be either by Hbox' Jiggs or Mango's Fox, though at that level it's player moreso than character (Peach is definitely not the best in the game... nor is Young Link
).
MK has no bad matchups either and is in many ways similar to Melee Fox in that everyone knows the matchup and he can be hurt badly for mistakes (chaingrabs, Snake). He's both good as a main and a secondary. If you want to bring Marth into the equation, the same more or less goes for him even if he is somewhat worse. He has quite a few matchups that aren't free, but he's not really held back by anything either and has had Europe on lock for years now. To me it makes no real difference whether 5 out of top 8 players use MK or Marth, not to mention you'd see even more Marth the further you limit MK.
It's rather interesting on this note that SSB64 Pikachu has no matchups he doesn't win outside of a few character-stage combinations that are even. In that game too though, even bad characters can get long strings. Smash has never been that balanced.
The French scene has a weird 2011 PR, which is why we don't want to publish it on SWF : it's irrelevant. Leon was, up until now, far above the 3 players that were right under him, but (and I quote him, here) he reached a skill cap he has yet to overcome in September/October. Deimos managed to catch up on him only after APEX. We'll see this weekend and next weekend if stuff turned around. Leon being so far above up until now is why I don't consider instinctively first place characters to matter much more than the rest of the top8, I apologize for that.
Eh, first place still matters even if you feel the best player is an outlier. Not to mention it's natural for people to catch up to him at some point... but it would be a problem if it's MK, who no one denies is the best character? I feel people are pretty much never satisfied anymore and are unrealistic in what is required to keep the character acceptable. It's not okay if he wins, it's not okay if he loses and he only has even matchups if they always beat him, otherwise it's just a fluke. I'm kind of exaggerating, but I feel people need to redefine what they expect out of top tier characters and see how realistic this is, also beyond the scope of Brawl.
We can still view rulesets as a solution to a problematic MK because there's clearly a matter with that guy
While your assessment of the three rulesets (Japan/Europe/US) isn't necessarily wrong, I think it's important to point out that at least Japan and most of Europe did not base their ruleset around MK. A limited stage list is what felt right from the beginning and is not a compromise if it's been that way since day 1. But yeah, MK has not been able to function well under large stage lists, which makes him once again similar to Melee Fox... most stages banned from Melee were banned because of him and he still has no bad stages.
The choice still seems to be between less stages or a ban and most of the world is going with the first. If you want to find a middle road, it'll be formed through arbitrary lines, but if it makes your region happy you can always do whatever you want.