Well, Ryu and Roy may not have been officially confirmed, but their inclusion is nigh-undeniable at this point. The files are in the game, and it wouldn't make any sense to wade through miles of copyright papers in order to get Ryu into the game and then just ditch him. There's no way Sakurai would do something like that. Roy is a Melee veteran, and the most popular one, at that. Pichu's hatebase is massive, Toon Link literally is Young Link, and Roy, while not terribly popular by any stretch of the imagination, is still the most popular out of the three. Plus, he takes 1/6th of the development time a unique character takes, so it's likely that the devs made Roy during a "downtime." Personally, I'm very excited to see how Ryu is going to translate into Smash, and I'm glad to see Roy back. I don't mind clones at all, and I personally preferred Roy to Marth in Melee, so I might find myself using him on occasion.
As for further evidence that the leak is real, the data miners also found several unused Kirby tracks buried in Smash 4's files, and it's actually possible for you to fight Kirby characters on Battlefield in Classic Mode. That never happened prior to Mewtwo's DLC, and it makes absolutely no sense. What this probably means is that the Smash 4 devs were making a DLC stage for Kirby, and they were able to add the music into the game, but not much of the stage data. So, in order to prevent the game from crashing when it tried to load the DLC stage, they added a "rerouter" of sorts into the game's code that redirects the stage loader to Battlefield whenever it tries to load the unfinished Kirby stage. Some people of have speculated that the returning stage may be Dreamland 64 because of the near-identical layout between it and Battlefield, but it's also possible that Battlefield was chosen as the go-to stage because of its rather basic layout. Anyway, that's all speculation, so I won't go too far into detail.
As for Mewtwo's tier placement, I'd say that you're about right. He's no lower than mid-tier, but I'd say that he has the potential to be upper-mid or even low-high tier.