All of this is really shoddy evidence.
He was the character with the most data, suggesting he was cut late in development. I think being planned for all three games is a favor to him. Greninja has nothing to do with Mewtwo, period. He isn't filling a "niche" that Mewtwo would fill. If Sakurai wanted to add both, he would. Sakurai being asked about the Mewtwo thing holds nothing either for or against Mewtwo, as he shouldn't be blabbing out info about the game. Gematsu has nothing to do with Mewtwo's inclusion, as the leaker never specified if those were all of the newcomers (which Mewtwo even isn't).
The CSS for a DEMO for Christ's sake means nothing. This was the Brawl demo:
Does that look anything like the Brawl Roster? Apart from the six at the top staying at the top, there is nothing to support that the final roster will look like or model itself after the CSS DEMO (and remember that the slots shrunk from Brawl's starting roster into more manageable spaces). Also, Jigglypuff being in, again, holds no water for or against Mewtwo. There aren't these "slot" capacities. Sakurai will add whoever he damn pleases and will make it fit on the roster.
Also, can someone fill me in on this Mario Theory?