What evidence do you have for this "outdated" claim?
The fact that so far he was correct with every character bar one (excluding clones), who was said to be considered by Sakurai at one point before replaced by Robin. Also, there was a placeholder spot for a Pokemon from X/Y, which Sakurai later confirmed existed prior to his choosing of Greninja.
If the roster was "outdated" (and thus, dating to before the game's development), why would it come in two separate leaks? In fact, wouldn't the older and more "outdated" one be more likely to come out first?
Brawl's roster leaked incrementally too, but there were valid leaks. I can't tell you anything about why things happened when, but the information included is more important than the timeframe of its leaking.
Plus, the first leak was only to leak the characters at E3 2013, not every character, though they ended up being partially incorrect about that.
Who is responsible for this leak, since we know that, even in-house, the roster was kept behind closed doors?
If we knew this, Nintendo would know this, and they'd be in a world of trouble. Just because it's kept behind closed doors doesn't mean it's impervious to leaking. Like I said, Brawl leaked too, and that was just as guarded.
If an outdated roster (again, dating back before development started) was leaked, why would it have waited so long to get out to us at all? If someone was going to leak it, wouldn't they have done so a couple years ago?
Maybe the leaker only came into possession of the information recently even if it's not current information. For all we know, Sal's source isn't even the original leaker.
If they have an outdated roster list that they're pulling from, why did the first leak sync up so well with the current announcement schedule?
The leak that said we'd get six characters at E3 2013? That didn't sync up with the schedule at all. I mean we didn't get two of those characters until a whole year later.
It's not "outdated". The first one was based around an in-house build for the characters commonly known around the company at the time. The second leak was just made up (as though "Gen 6 Pokémon" didn't give that away).
That's quite a lot of assuming considering how much evidence you have for those claims. If anything, the X/Y slot corroborates with the info from development, where the character from X/Y wasn't chosen until later, like I said above.
In any case, I'm not going to debate the validity of the Gematsu leak here anymore, this isn't the place for it.
One trailer to reveal the character, yes. But then you have to reveal stages, enemies, items, Assist Trophies, etcetera. All of these extra things that characters in-game have. Nintendo is running out of time to reveal those if they reveal a character tied to a new IP.
Those parts don't have to be revealed pre-release, the character is what's most important. How many enemies, items, ATs, etc. have been revealed from Punch-Out, and that series was revealed months ago? As far as stages go, they can be revealed in the same trailer, like Greninja or Pac-Man's. In fact it's probably pretty likely they would be if it's a new IP.