NOVEL WEB Chapter 69: *About Solomon*
I am just gonna clear something about Solomon.
"How did Angra Mainyu summon Solomon?"
Grail can choose whichever Servant to summon according to Master's personality. Angra Mainyu is part of the grail. She can summon any servant as long as they are recorded in the Throne of Heroes, not the Divine Spirits, of course. She doesn't need Catalysts.
"Why has Alaya/Gaia not noticed Angra Mainyu's actions?"
She didn't care when fourteen servants were summoned or Angra Mainyu became a mess with Sakura. Nasu himself said Grand Servants will be summoned when half the world was submerged in All World's Evil.
"How did Solomon become a normal Caster?"
Solomon has the qualifications to be summoned as Grand Caster. However, he can also be summoned as a normal Caster. Marisbury summoned Solomon as Caster in FGO timeline. He is strong but not as broken as his Grand Caster version. He is still limited by his homunculus vessel.