Might as well go down the whole list. Billy if you're reading this and wanna make a video about the insiders and those in the know in reference to the leak, go ahead. This information took a LOT of time to compile.
Gamexplain. 3 of the 6 members of gamexplain have come out saying very confidently that it is fake. Derrik who just hate's leaks in any capacity, Jon who's proven to have industry connections made a bet saying that if the leak was real he'd sit in a dirty puddle (lol), and Andre who says there should be a pokemon. He got very angry about the leak thread on reset era later, comparing it to a trump rally. That got some backlash but since then he's remained silent on the matter.
Spawn Wave. He runs a gaming news channel with an emphasis on Nintendo. He has some mutual's who are in the know. He personally doesn't believe it. On account of Incinroar's absence, and Geno and Banjo's presence. He agree's heavily with Andre seemingly, and commented that the reset era thread would be a thing of beauty before long.
Roger's base. He runs a One Piece and Nintendo hybrid channel. He's friends with several of these people and he was a known supporter of the ESRB leak when it originally hit. This time he is taking the stance of team fake. He's not an insider by any stretch, but given that connection to gamexplain it's not a stretch to say that they told him Incinroar is in.
Liam Roberston. a reporter who has a TON of connections say's that it's fake due to his own sources. However, he has acknowledged the possibility of the rumors he has heard being false, as it has happened in the past. He said he'd willingly take the L if he was wrong, as he put it. He personally theorizes that someone had assets from the leakers previous work place were used to create the leak. He has been very defensive about his stance on it being fake, and call's most of the counter arguments straw grasping.
NGameTheCube. He's not an insider but he is also very well researched, he runs a youtube channel covering Nintendo and leaks. He has predicted a lot of the characters, like Ridley a day or two before the reveal with confidence. He has been very vocal about his support of the leak, and how he thinks it's completely true on every level. He's known for perpetuating the idea of Steve being playable this time around, which came from some leakers.
Spooky Mystic. This is a known leaker who primarily operates within the Kingdom Hearts and Persona communities. He made the claim that the leak is real yesterday. But today further clarified that he had been approached by a ton of people saying with absolute certainty that Incinroar is in. He said that something strange was going on, that smash leaks are uncharted territory for many, and that all the insiders are on different levels of knowledge with tons and tons of conflicting reports. He also said t one newcomer trailer was gonna happen, and that it would contain two characters. And ended off by saying he wasn't going to touch smash again.
Alex/Nintendo Life. Nintendo life is a news website with tons of connections. They produce reviews too. Alex is the face of the youtube channel. He made a video on the leak saying that it was fake. Saying that it was a bit too wish-list like and so on. I myself actually contacted him as he mentioned sources at the end of his video. He told me that he wasn't willing to give out sources (obviously) but stated that said source has never been wrong in the past.
Artsi Omni. He's again, not an insider. But he's a very infamous fake leaker at one point himself. And he's got some background in game development, and is also friends with Gamexplain. He seems to think it's fake, on account of differences in battlefield. But has remained neutral on the whole thing throughout.
Vergeben. The currently most infamous smash leaker or insider. He said that it's fake on account of not having Incinroar. Making a meme joke about Incinroar on life support and posting it on Gamefaqs. He got extremely angry when people made the suggestion that Banjo and Isaac were not in, telling them that just because this leak was in his opinion fake, doesn't mean that Isaac and Banjo and whoever else aren't in.
Leaky Pandy. This is a twitter account that posts rumors and information regarding upcoming games and the like. They've bowed out of smash leaks and have built a bit of a rep for being unreliable. They've gotten direct dates right in the past but missed the content within them. Anyways, they didn't want to comment on the banner leak, but when Liam Roberston confirmed that the leak's person was in fact a real guy who worked at a real print shop, they simply wrote "RIP" They then retweeted Push Dustin's posting of the translation of Marina's statement. They then later commented on the toxicity surrounding the discussion that Andre alluded too.
Stealth_. This is a pretty popular twitter user and he does market research. He's been a guest on various podcasts in the past as well. He's not really an insider, but he's alluded to thing in the past that turned out legitimate. For instance, he firmly believed that a new DK character would be present in the game, and some time late reply we got K rool. He firmly believes that the leak is fake, saying that some nintendo employees must have had a laugh. After that though when the sprout item was identified as potentially from golden sun, he cast out support for Isaac, an action that he has taken several times in the past.
Ryan Brown. This is another reporter. He's fairly close friends with people like Laura Kate Dale, another infamous leaker who was behind the magnum opus that was the switch leaks. He commented that Genuine leaks have happened in the past and that they aren't out of the realm of possibility. He personally wants the leak to be true on account of being a mach rider supporter.
Nintendo Voice Chat. This is a podcast hosted on IGN. Gamexplain has been on with them before. Being associated with IGN it's fair to say that they'd be in the know. That having been said, they've seemingly cast support in favor of the leak, discussing it positively in episode 430 of their show.
[Update 1]
Papa Genos. This guy is a smash leak youtuber. He originally created his channel in support of Mach Rider, which is a character that he felt was severely underestimated. He has made videos covering various 4chan text leaks and image leaks leading up to this point. He claims to have a source, and this source has a source from within Microsoft. And said Microsoft source was aware of Banjo, all the characters currently on the banner, in combination with Incinroar. Because of Incinroar's absence this source is very confused at the current time. I contacted Papa Geno's personally on discord to ask about the nature of the source. He stated that he obviously wasn't willing to give up the source, but that they came to him, and that if I were to know the source i'd probably believe him. He currently presents the theory that either A, Incinroar is DLC, B, Incinroar is leak bait, or C, the leaked banner itself is leak bait. He firmly believes that this leak isn't fabricated due to his source, and that it has to have come from somewhere official. Regardless of it's true nature.
[Update 2]
Emily Rogers. Emily Rogers is a ridiculously promintnet Nintendo’s leaker with a very strong track record. For instance, she leaked many years in advanced that breath of the wild (then called Zelda U) would be the biggest Nintendo video game of all time. She did comment prior to E3 that smash switch’s roster would be “unbelievably amazing” however, After the switch leaks, she bowed out of the leak game for the most part. She replied to Nintendo’s everything article on the initial grinch leak, stating “I wouldn’t buy what that grinch leak Is selling” I reached out for comment in a reply, if she reply’s or adds anything, I will amend further. It’s unclear if she thinks the leak is fake based on sources, or if it’s her personal take away.
A very diverse range of opinions are represented. And if I misrepresented anyone, or wrote something in an awkward fashion, i'll amend my post as needed. The common theme here is that people think it's fake on account of Incinroar's absence. Or think it's real on account of surrounding evidence. It's a mess, to say the least. I'm not sure what to think. I've only managed to keep tabs on all of these various people and sources at once just on account of surfing the whole damn internet at once the day this leak hit.
[update]
I personally trust Emily Rogers more than anyone else on this list. Is she claims to have information, and this isn’t her personal guess, I will be team fake.