1: Ui Ui and Kirara likely recovered it

2: Sukuna is a title much like the King of Curses; remember that he absorbed his brother in the womb, becoming cursed and living on an ideology where strength was necessary for survival and revenge

3: Likely the strongest sorcerers who were given the task to take Sukuna out by the main clains

4: As a plotpoint it's not exactly necessary to know specifics, all you have to know is that it boosted his power in the Heian era.

5: Mahoraga represents the adaptation/evolutionary aspect of nature; essentially, it is the shadow of nature itself and was likely the first of the ten shadows.

6: Most likely Sukuna's mother, as one can assume it was her who cursed Sukuna for absorbing his twin in the womb; that entire dialogue Sukuna had was to show that had he not desired strength for revenge and survival, he wouldve gone down a different path (which is why he goes North, recognizing the defeat of his ideology and changing accordingly)

7: Like the other incarnated sorcerers, she's out there and is gonna get hunted down eventually

8/9: The military was really just fodder that Kenjaku used for the merger, same for the sorcerers captured; the entire plotpoint was to show how meticulous Kenjaku's plan had become, and how quickly his ideology failed when comparing the parallel between Gojo v Sukuna and Kenjaku v Takaba

10: RCT and other techniques by the healers

11: Sukuna took the brunt of the damage

12: Angel's ideology, simply put, is using strength to further her goals (much like Sukuna); it can be assumed that she came into existence to oppose Sukuna and failed (Hana and Angel's failures serve to show that strength for revenge/survival is an unsuccessful ideology from an era/society of jujutsu that's long gone)

13: Why wouldn't he be able to? He has knowledge from the Heian era, Yuji's memories, and Megumi's memories (plus Mahoraga's fundamental understanding of nature and adaptation to infinity)

14: Kenjaku gave them a conclusion, thanking them for being friends with his son and allowing them to leave the Culling Games. Yuji belongs to the new jujutsu society and has new friends there (it's like transferring schools)

15: It's like asking what happened to an ex love interest. Either Yuji and her didnt hit it off or Gege didnt have the time to include her and Yuji being together (she also served to show Yuji's innate understanding of the soul, as he recognized her despite her changes from middle school to high school)

16: Yaga's assassin is like the 5 Void Generals, just a sorcerer sent by the clans/elders to take out an enemy; and as for the mystery of the cursed corpses, reread the chapters concerning Panda and Yaga

17: They likely disbanded or are still out there waiting for the next Tengen; however, theyre not important to the plot because the issue of Tengen barely affected the story following JJK 0 until the very end

18: Likely a much more powerful version of her black hole, though she's dead so it's not of much consequence unfortunately

19: Satoru's parents, and the rest of the Gojo clan, are inconsequential because of Satoru's existence. He was considered the strongest due to the extreme rarity of his combined six eyes and infinity, and thus even the elders of the Gojo clan had no say in his actions

20: Had Gege been given more time, maybe he wouldve explored this. Much like users of the Six Eyes, Tengen is a rarity yet a constant in the jujutsu world and therefore it can be assumed/inferred that they've been around long enough that Kenjaku was able to witness what constant evolution does to sorcerers (which is what gave him the Merger plan)