So, it appears that the recent leak coul be legit. A friend of mine made a long message on the french ssbu FB community, explaining why. I'll try to translate it for you guys. (Credits to David Dian on FB, alias Ray Sober elsewhere)
"Let me explain.
1) Pichu's ear:
Obviously everything happens on a pre-e3 build. For reasons we do not know, on the video of e3, some characters are entirely in the diamond, others come out entirely, others are cut. Pikachu for example also has his ears cut off. Obviously they change a lot of graphic things later (see the modified artwork, the Mappy pixel art, etc) so quite possible that this icon of pichu was deliberately cut to properly make the scene of the video of the e3 which present pichu, zooming on his icon by cutting the ear at the similar place. These late graphic remakes would also explain the absence of Ken's icon. (Personal note : It's actualy because of the zoom, the icons get transparent when the characters could be hidden)
2) the timer perfectly stopped at 2.30.00
It's an early-build, obviously during a debug mode. They can do what they want. Even make the timer freeze at 2.30, something quite common during testing phases.
Now that I have enumerated the counter arguments, here are the arguments:
1) The situational randomization of the screenshot
One of the well-known flaws of the human is that he can't manage, in a voluntary act, to re-produce natural chance. Artistically speaking, a person goes without realizing to make certain artistic choices by patterns rather than chaotically.
The leaks we are used to, when it comes to screenshot of ingame characters like this one, by default always have screenshots of characters in simple positions and / or "clean" places. Like idle mode, full jump, well positioned on the stage quietly, etc.
There, we have the foot of pacman crossing the ground, Ken's foot levitating, etc. Someone who would make a fake like that with 3D models, unconsciously avoid this chaos, and take a screen with perfectly positioned characters.
2) THE STAGES
I put it in capital because it's the most important part.
Look. Look on the internet whether on the build of e3, direct smash or anywhere. You will NEVER find the splatoon stage without paint scattered everywhere to decorate. Obviously in this build, the painting is not present or activated. A fake leaker would never have made that mistake if he had to remodel himself the entire stage.
With all these elements, I'm 99.9999% sure it's a real leak."