The MS isn't really canon as it's not in the novels and it contradicts a certain element from them (he was proud of Itachi and members like Shisui but at the same time was jealous of them due to skill... should he be jealous of younger shinobi if he has his MS hidden away?)
But, I guess it can be canon because that can not be a "contradiction" depending on how you look at it. He could have just been jealous because Itachi progressed quickly in the novels but not from an overall power standpoint- just one of talent. It probably just is skill as being better at things like shuriken jutsu or being more stealthy isn't the same as brute power.
I'm going to consider it canon as Kishimoto has been giving his blessing to the novels (they gave some novels with his signature in them to fans a few days ago) and he's been at some promotional events for the anime arc.
This would also make the Uchiha looks slightly less dense. As in the novels, they knew Itachi was not going to help, that Shisui was dead, etc but still wanted the coup- when they did not have the power to pull it off. And in the novels, it reveals Fugaku isn't for the clan ideas but he always denies this when Itachi asks him- they made him much more sensible in the anime.
Whereas here, Uchiha don't know of Fugaku's ability but it makes more sense for Fugaku to not go for negotiation and instead still want the coup if he thinks his mangekyo might (okay so still not entirely sensible, but better than the novel) be able to make a bloodless coup. But even this is somewhat stupid as his points were a) Itachi could assist him alone... two people are not enough, b) The anbu would assist Itachi... nonsensical, they aren't for Itachi but work for Hiruzen/Danzo, c) The clan is too powerhungry, he can't stop them... then why be a leader?, and d) If they take out the higher ups then all is fine and dandy... no, people would not want an Uchiha Hokage unless it was Itachi, they suspected the clan for the Kyuubi attack, and as such they'd have to take control of all shinobi... how can 2 people subdue 1000s? There is no mass KA, Shisui's eye is deactivated for ten years at this point and Tsukuyomi is not long-term for such a process. Should have just gone for negotiation and the peaceful route, not vouch for the coup by saying it can be bloodless when they live in a ninja society of assassins and have no genjutsu capable of such a feat anymore.
but its all similar to amegakure childhood arc, not in manga, but cannon.
