tbh i think you should reread the culling game arc cause it answers those questions you have…
I've reread it 3 times and it really doesn't my dude. He's not a good storyteller.
Yuuji’s past doesn’t really matter to the plot since he lived as a normal kid up until he ate sukuna’s finger. Kenjaku was a sorcerer way back whenever, being part of the Kamo clan (blood art) and he was gathering sorcerers to resurrect so that the culling games could happen. Reason why he set up the culling games is cause him and sukuna made a deal while also giving other sorcerers the chance to fight one another. the merger was to happen in case sukuna was able to dominate, allowing all of humanity evolve in a way it has never before. Culling games was part of the plan all along. Megumi being a vessel makes sense because the manga explains how the previous shadow user and 6-eyes both killed each other when fighting. Sukuna took megumi’s body to again the extra advantage of having the shadow technique to fight against Gojo.
Idk if i’m right or wrong, i wrote it all from what i remember from reading the story. Either way, if i can recall facts about what you questioned, then that has to mean there was something in the story providing the information.
nonetheless, the ending may seem unsatisfactory since the build up the climax was crazy, then it all cooled down pretty quick. also, do take into account that allegedly, gege’s editor made either a bunch of changes to the story or made some major changes that changed the original story.
So we are just going to ignore everything about yuuji's past and the brain guy and him being the main character for a shitty ending. Culling game shouldnt have even happened, megumi being a vessel makes 0 sense. Gege you are a terrible story teller. I really hope the sequel doesn't happen.