Hajimekata - Profile

There only one sub war that was describied more or less Fate/Labyrinth(novel). Kirei can take part in it because it doesn't destroy lore in any form.(this war creatied by vampire). There also the best legal yandere loli of Fateverse Manaka Saijou will be there(in soul only), and Kirei who is outside of Root would likely atract her attention.

Yep, but you can try sub-holy war, like Fate strange fake, in Apocrypha after third war there were many of such wars Keyneth died in such war. Maybe there won't be many of such sub-holy grails but they do existied most likely.

I think he can summon: Foreigner like Abigail Williams(His nature and his personality) the best match Lancer Vritra good mach Assasin maybe King Hassan Ruler Shirou Amakusa and for laugh Caren Berserker maybe Raiko (because of dual nature) Archer maybe he is good mach for Arjuna

Shinji is still Shinji in the end(even he isn't truly Shinji), everyone forget that Leo hasn't used his power of the beast directly and there is still Book of The Law with his true power. Shinnji created new world is like giving Leo a green light to use his true power and his Noble Phantasm. He also took all Agro from Alaya and Gaia.

Domina Coronam: Ten Crowns A skill on par with a Divine Authority. It has been said that the ten crowns indicates the ten evil kings and the seven hills indicate the abominable city of evil. It is unknown what kind of Authority it is, but that power's origin lies in a past that even the King of Heroes, who was supposed to be mankind's oldest, cannot reach. On a simple look, it is a Skill that turns received injuries and occurred events, that make all results as if "nothing happened". As a matter of fact, BB is invincible as long as she has this skill.(BB and Goetia(had this skill as King of Humans for some time)) Domina Coronam: Crowns of Ten Rulerships worn by the Beast of Babylon converted into a teacher's cane. This is due to the theory that the seven-headed beast is male, which opposes the theory that it is female; its true nature is a phallus (an erect rod). The symbol of the king of the world who was granted power and authority by a great dragon and, during a period of 42 months, spat as many arrogant words as they desired and had received the right to desecrate all things. The beasts heads are the seven hills of the Roman Empire; Capitolium, Palatium, Aventinus, Esquilinus, Caelius, Quirinalis, Viminalis. In other words, it symbolized the Roman Empire itself, and those ten horns were symbols of Roman emperors; Augustus, Tiberius, Caligula, Claudius, Nero, Vespasianus, Titus, Galba, Otho, Vitellius. About "Seven Crowns" Nero does have such skill as Summer Nero at C rank, it give her ability to ignore class restriction and freely use all her abilities, it also reference about her ability to become a pseudo(because Leo is true Beast VI) Beast VI as Whore of Babylon or Mother Harlot and I think what Manaka was trying to create will be Nero Alter's Noble Phantasm.

I have a question how she or he know MC's name after all, Leo never said his human name to them. Merlin isn't strong enough to look though Leo.