Heroic spirits born from fiction and hero of fiction are different things. Heroic spirit being born from fiction requires a specific reason to be on the throne, such as Nursery Rhyme was able to become a fictional heroic spirit of children due to being the base of children stories, a general term for children picture book. That's how eventhough she is a fictional thing and not a legit heroic spirit, she's still in there as a fictional heroic spirit.
First of all, I am talking about original installments of Fate series, namely Stay Night and Hollow Ataraxia, Zero to a degree.
SN talks about fictional heroes in the opening monologue, do you really believe this was foreshadowing of something which would come half a decade later?
Secondly, I need to see anything elaborating the distinction between these two types.
It wasn't the Grail, but some sort of database for humanity (the Root?).Easy, grail picked out some random wraith who just happened to have close abilities to "what was described in the
Nasu: If a Master wished to summon Kojiro Sasaki, he would be unable to because Kojiro Sasaki did not exist . Like the story stated, if there were a database for humanity, then Assassin was the piece of data that matched the parameters of Kojiro Sasaki.
Pay attention to the bolded words. The problem isn't that there is no fictionalized version of Sasaki, the problem is what he didn't existed.