For what it's worth, unless we're dealing with an incomplete character select screen (but why would the screen be incomplete if the backstory of the leak is true?), I find it hard, if not impossible to believe that the real, complete character select screen, given the standards set by Sakurai, would:
a) have Yoshi splitting up the Mario franchise characters;
b) put two spaces (in a row of nine, so that's asymmetry) between Bowser and Bowser Jr.;
c) not have Dark Pit grouped with Pit and Palutena*;
d) not have Dr. Mario grouped with the other Mario characters*;
e) not have Lucina grouped with the other FE characters*;
f) have Shulk grouped with the third-party characters.
(* unless you assume this is a 'clone corner', but in all the other games, clones were still linked with the characters they cloned, horizontally or vertically. Besides, shouldn't Toon Link belong in the same corner if that's the reasoning?)
I could believe the roster (though having Dr. Mario back is - pun totally intended - a tough pill to swallow), but this select screen lacks the symmetry and the logic that I believe someone with Sakurai's attention to detail would put into it. Too many red flags for me to believe.
So basically, my opinion is that this is either a real but incomplete screen that will make more sense with more unlocked characters or the work of a faker who's much better at faking renders than he is at putting together select screens with any semblance of rhyme or reason.