What is even going on anymore? I can't keep track of any of this. I have no idea who Mystic or Stealth are, and now people are talking about someone named MTP in the general discussion topic? Anyone have a tldr for who these people are and why I should care?
Mystic a.k.a. Spooky Mystic has worked in the past as translator on multiple projects, mostly JRPGs, that come over to the west. They jumped in on some of the Smash speculation recently and apparently has sources that have told him that the banner leak is fake but several characters being in is real. This was mostly just today with that, and he just had his twitter profile made to private. All in all, a very minor insider.
_Stealth has been around for a while. Talked about and leaked Simon Belmont before Vergeben did and several other things related to Smash and other games. He has also gone on record now for saying the banner leak is fake and has heard sources telling him notably that Isaac is not even after originally saying he was.
TL;DR insiders are just as confused as you are, there's no reason to trust them in the least. Just move on.
As far as MariothePlumber, a.k.a. MTP, he's taken credit for being the original Chadondorf poster on 4chan and also foreshadowed that this grinch leak would land at the anniversary that the ESRB leak did. He's very bombastic just like the Chadondorf post and curses like you might expect from the original grinch leak. He also is from Quebec so there's some vague connections there to perhaps being involved in this stuff. He's mostly a youtube content creator and rarely posts on twitter except for just recently. Basically, nothing concrete.
And there's nothing that really happened overall. Basically people saying things that can't be verified and right now aren't worth the effort to understanding the origins for now. The leak is still on neutral grounds and still awaits information from Nintendo or a Nintendo Direct itself.