I've had a really hard time describing my thoughts on "insiders" lately without angrily cursing but I'm going to give it a shot here.
Now I have no doubt that there's a sprinkle of truth to what some of these people are saying. Through all the white noise and random shouting at the sky we have managed to find out Ridley, Simon, and Isabelle before they were properly revealed. It stands within reason that at some point, someone had access to the game and saw those characters at the very least.
You'll notice, though, that this is essentially where the information dries up. Ken is another character, so is Incineroar, and even Incineroar is a complete coin-toss going into Thursday. We have gotten no new, concrete, leaked info regarding characters in this game since the end of July/early August.
You know why that is? I have a hypothesis. One guy was the genesis of all these leaks. One guy who told a couple friends who told a couple friends and so on and so on. The so-called "insiders" have almost certainly not seen anything substantial from this game. It's a very long game of Broken Telephone. Here's a simplified graph:
Our insiders? They're the people who have heard from 1ab and 2bb. Or maybe they've heard from 1abbba and 2babba. They are nowhere near the top. If they *were* somehow endowed with some kind of insider knowledge, simply talking about it would ensure they never hear anything like that again. The idea that some of these people have been sat down to watch a Nintendo Direct early and are then allowed to talk about it, or reveal which characters are playable and Assist Trophies is laughable. People have lost their jobs for less, let alone press passes.
Stuff like dates, titles? Yeah, you can probably rely on an insider for that. Stuff like playable characters? It's really doubtful. I wouldn't put too much stock into someone saying Geno of all people is going to be an assist trophy, for goodness sake.