Kashin koji is based on
"Koji Kashin shares his name with a folkloric believed to have lived in the late , though his is debated. He is often depicted as a white-haired, bearded, elderly man in robes, and is said to have used his illusions to deceive and defraud prominent rulers such as ."
So it's a character that does illusions and tricks people. Kashin koji has the physical description as listed in this but him doing illusions and defrauding people dont fit.
A clue that hes not truly a clone of jiraiya is flame of samadhi that's not a technique in his arsenal.
I'm not going to say who kashin really is but I'll give a clue hes a character in naruto that is known in taking jutsu to a different level and would know fire release.
As for amado its more of a hunch but saying he created a jiraiya clone its someone that he has a relationship with him. Like koji amado is another character we know and not truly a brand new one.
And hes been blatantly shown plotting a scheme in franchise.
As for isshiki thinking hes a clone we see amado has been one step ahead and in isshiki arrogance think he knew everything.
As for jigen taunting koji that amado is using him either he reacted the way he did to trick isshiki or might mean hes brainwashed by amado and forgot who he really is.
As for the amado daughter story think thats true but it's not a character in original naruto.