Here goes headcanons because I don't want to bother with doing analysis.
1)So you know how Shade entry in Hunter's Journal is immediately available? The wording of the entry is vague enough for my interpretation of it.
Shades are as far as we know are related to regrets. Now imagine Knight, still very young and not long has passed since they hatched. They got out of Hallownest and someone finds them. Finder goes if not friend why friend shaped and adopts or befriends Knight.
Knight is still young and doesn't know of their immortality yet, so years pass and Finder gets old and older until Finder dies.
Knight is immortal and they felt healthy the entire time, Finder's death affects them badly. They continuously think what they could have done.
Regret wells up within them and they cry Void. From it raises their Shade, their regrets manifest and a life they could never return to.
They don't exactly understand what is going on, but they can sense they are not whole anymore and that their insides are like broken jagged shards. They understand that Shade must die for them to feel whole at some level, so they attack.
2)A funnier headcanon would be Knight being a scholar. It is quite clear in the game that Knight is magically knowledgeable and talented. They do a lot of random things that player really cannot interact with that is magical in nature.
Most basic one is their ability to devour Soul, how do they do that? Did they learn it or was it something innate? What does it feel like for their opponents? Etc.
From Silksong we learn that not only Dream is related to Memories but Soul is also related to it which we learn via Binding process and some lore drops regards to it.
Even outside of Knight's causal Soul eating there are particular points which stand out, Spell Souls, which to be clear carry the knowledge on how to manipulate Soul to cause the spell not the ability to use the spell itself. Knight can already manipulate Soul and stranger energies at game start, Spell Souls wouldn't grant the capability but knowledge.
So, with all these Soul eating and Souls including memories and Snail Shamans(from what we learn from silksong) and Soul Sanctum commiting crimes against bugkind I imagine Knight has a lot of unrelated and worrisome trivia on regards to Soul.
So, after they finally figure out Hunter's ugly as sin handwriting they see Hunter mention he really doesn't know what Soul is. Knight reading this just travels as quickly as possible to Hunter to trivia dump and educate him.
3)Knight can teleport. I mean they can teleport even outside of Dream Gate. Grimmchild, Grimm, Nightmare King, Pure Vessel, and Radiance all can teleport. They are beings closest to Knight in terms of power, if they all can teleport only reason Knight wouldn't be able to would be game mechanics.
In a similar vein to how Cornifer can access to places he shouldn't be able to or how Flea Caravan reach Pure Lake despite nothing in map points toward anything that is viable. Which is explained away with game world is bigger what we see.
4)This is more an opinion but I do think comparing size of Hallownest and Pharloom is a rather pointless endavour. What is available to us in the games more reflects how much money and time TC had access to than anything lore related.
5)If Knight and Hornet had to design wings for Pure Vessel Knight's design would be something that is powered by countless souls and kinda hurts to look at, Hornet's design would look like fluffy steampunk, don't ask me how that looks.
6)Ritual of Nightmare Heart we see is nowhere near the standard version.
We already know that Knight can and does modify rituals of other people from Pantheon of Hallownest. My general argument for this is the Void Gate that intertwines with Pantheon entrance and more importantly existence of Winged Nosk.
Nosk originally imitates Knight in shape because it cannot read Knight's mind, and assumes that Knight would follow something in a similar shape. This isn't something special to Nosk. Pale King was unable to sense any thought from Pure Vessel, and Knight also fails in pulling something outside of minds of other Vessels with Dream Nail.
Under these conditions I am really doubtful Godseekers would be able to succeed where others failed yet Winged Nosk is in the shape of Hornet. Only logical explanation is that Knight modified the Ritual for their own purposes, which also required giving a personal touch, fear of killing their little sister.
So, I don't think murder of Nightmare King is an actual part of the Ritual, there's not many beings that can kill them and I am doubtful number of beings we knew that could changed with release of Silksong.
So, murder of a Higher Being can't be the standard procedure but a modification Knight did with potentially interesting consequences.
I guess this many headcanons today is enough. Maybe I will post more tomorrow.