Even if Magicant is revealed, there wouldn't even be a chance of it matching the picture, for the simple reason that stage reveals don't show us what the icon on the stage select screen looks like - thus we're still arguing about the Tomodachi Life icon. However, a Magicant reveal would make me lean towards real anyway, since most of the speculation about the new Earthbound stage was for Saturn Valley or Giygas' lair. I can't remember many people talking about Magicant.
The most likely PotD to deconfirm the leak, I think, will be a 3DS stage that's not included on the leak, especially one from a major franchise (Kirby, DK, Metroid, Star Fox,) because that'll block any arguments that "this stage just wasn't unlocked in the leak."
Exactly. Well put. The other option is we get magicant reveal, and then second screenshot is the Stage select screen with the exact magicant portrait. That would confirm/deny it
Sakurai seems to be going even more retro for this game, especially for 3DS (game boy stage!), so I honestly wouldn't put magicant out of the question.
Color game 15, pac-man, mega man, game boy, and, if real, duck hunt dog, are all pointing towards a smash bros that is really really retro, while still embracing the modern (greninja, wii fit trainer, awakening). It's like he's broadening the horizons even more this time around.
Also, as I'd like to point out again, the Stage select and character select screen in those leaks are clearly missing spots, so I wouldn't ever count the leak as being final roster and final stage select, even if the rest are DLC.