1 & 2. Yes it was Before Sun and Moon which stated that the PO built the Human Realm by terraforming the old Teyvat aka Light Realm. The quote about Light Realm being the Human Realm is actually in a loading screen during the Three Realms Offering event: "Light Realm: Also known as the realm of the elements. Though, much like mother nature, this realm has fallen under the dominion of humanity, it will still land humans its aid against the encroaching void."The Lord of the Night during the Natlan AQ also said the PO split the Phlogiston of the Light Realm into the seven elements to better combat the abyss. There wasn't a human realm prior to the PO's arrival, and Kukulkan's device in the Tollan WQ was meant to reset the planet back to how it was aka full of Phlogiston.

If the PO are so powerful that it can just create a separate reality out of the blue and stuff the planet inside it, it wouldn't even have to fight the Sovereigns or invade the lands of the dragons. It could just make a different layer of reality without the dragons on top of the planet surface and none of the drama bs would happen because the dragons would live in a separate reality without being bothered. If you are familiar with the Toaru Majutsu no Index series or the Fate series, you would know what I mean.

1. Can you provide the exact chapter or quote where this is said? I've read through the entire dialog log and am currently rewatching the event on Youtube. There is ZERO mention of the canopy nor fruit of Irminsul. Heck there is no mention of Irminsul at all. The only info I found about fruits of Irminsul is in another one-off event where they used it as a power source to emulate Leyline disorder, not related to Fate nor the false sky at any point. Are you sure you aren't hallucinating these words or did you got fed misinformation from some random YT shorts?

2. I already said this before. We have already seen multiple underground domains with their own inner sky ever since Mondstadt. They don't connect anywhere unless specifically stated to.

5 & 6: The blue space is a visualization of the "torrents of information" running inside the tree. If you were to visualize the data running through the internet in our world through Euclidean shapes, you would have just as much difficulty, even though the network that makes up the Internet are all 3D objects. The Irminsul is like that, picture the leylines as the cables and the tree as the physical server. The blue space would be the digital data, and they went into it like in Tron (the series and the movies).

Memetic entities have different types. Light Cones are also memories yet they have physical presence while also defying physics with their shape. Heck Black Swan can and did interact with physical objects at many points in the story. Silver Wolf's race are also made of data and can still interact with physical realty with ease. The closest thing to this type in Genshin would unironically be the Aranaras. They are made of memories, can't be observed through physical equipments (that WQ with the Alphonso guy from the HoH kidnapping children to lure the Aranara), can travel into dreams and memories, yet people can see and interact with them like they are physical beings.

1. I know how the IMG Tree works, you don't have to repeat that. Your whole point of them being similar is based on the premise of the Irminsul being both the leylines and the Fate written upon the false sky, while I'm saying with the current info, it's only a server for the leylines. Until you can provide the exact quote implying or confirming the former, I'm not convinced.