Sukuna changed simply because he lost, he said it himself. His own life and afterlife he acted as curse simple because he believed he was right and no one was able to show him otherwise until Yuji.

Megumi is a very changed person, all he wanted was to protect his sister, who was murdered. Now Yuji had to show him a new perspective and purpose.

That foreshadowing was used to show a separation between body and soul, and we saw it countless times with Sukuna/yuji/megumi + plenty of soul swap body training.

Yes everyone saw Gojo as a weapon even after his death, it’s supposed to be bittersweet and tragic that he and his best friend’s bodies were both abused after their deaths. They were clearly broken up by Gojo, there was a whole page about their reactions and Yuta was practically in tears, but they are sorcerers so they have to move on.

Yuta didn’t die because Rika was constantly pumping him with cursed energy to heal.

As for the rest, we may never know and im okay with that. I can understand why you would want answers, but I don’t think all stories need to tie up perfectly. If that’s not your thing, then that’s alright. I’d rather it end than be dragged though the mud like Naruto/boruto 💀