This isn't about DLC, this is about characters already in the game, hidden by a system link. The roster is supposed to be equal, and they would be taken into account when making both versions.
The Wii U roster is unlikely to undergo any changes, with how detailed it was. Everything they've shown us so far has appeared in the final game, why should this be an exception?
Also, the mode was already in the game, it was just enabled with the patch, alongside the rest of the online modes. Said mode was featured in the SSB3D E3 reel, and the update takes up almost no space. The whole "patch for online" thing was just so people wouldn't interfere with testing if early copies were leaked.
Mii Fighters already prove that they could just be a menu full of characters. The CSS doesn't matter at all here.
It's only proven fake if we find a definite real leak with the hacked coding of the game and we find no characters but the obviously cut Ice Climbers(and maybe Chorus Men, for the same reason, technical issues). The other one is a bloody fake. All of the names are inconsistent. They should all be Japanese proper translations of each one at best.
What Wii U Roster? We haven't seen anything but a tiny snippet of it way back when. There are zero leaks of what it looks like. The 3DS character order and Wii U character order have zero guarantees to be the same. We only know it'll have the exact same playable characters and nothing more(and that's 100% assuming it doesn't have exclusives we can add to the 3DS later. But we know all of them will be on the screen. Mii's may have 3 different slots now. We just don't know what it'll look at all).
Nobody knows what the final Wii U game looks like. They're not the same damn game at all. Since we have pretty much zero information, we can't find anything strong against this idea whatsoever(namely because there's zero evidence to suggest it's impossible. And real evidence, not severely old stuff that has no bearing right now).
Give it up. You're wasting your time here and you've provided quite literally no reason for anyone to actually find this unbelievable at all. Try your luck with how a character might work or something. You might be able to do something there. I would suggest doing some research in the leaks group, since you're quite unaware of the many factors in development, and how heavy of an influence Miyamoto is on games. He's always had influence, full stop. Him allowing customs to happen but make connectable unlocks can easily be a compromise between him and Sakurai. See how easy that is? Yeah.