Hello everyone! It seems I need to clarify some things.
First things first, how did Sayaka know the Rinnegan?
It's because everyone bloody knows about the Sage of Six Paths story but it is seen as a children's fairy tale by most - and this is canon, not an AU thing.
Jiraiya knew its appearance so either
A) He got the illustration on some text in a mission
Or
B) the Great Forgetful Sage described it for him or the toads themselves had an illustration of it.
Besides I honestly believe noble clans would have an illustration of it, probably one some scroll, because oral traditions when it comes to nobles often turn into illustrations and texts in real life.
In real life there are things like that, wanting to preserve cultural heritage and all that.
Beyond that, Black Zetsu before Madara awakened it, probably had some texts about it scattered so that an Uchiha or Senju could know of it (twisted version of course) so that the reincarnations of Indra or Ashura could pursue it (I heard somewhere that apparently BZ tried to awaken the Rinnegan in Senju's as well).
Which most likely leaked as a fairy tale story to some noble clans as well.
Plus, Nagato also seemed to hide it with his hair before the whole Hanzo confrontation, though that could be because his eye is clearly a dojutsu so maybe he would've become a target but who knows.
Now, the thing with Indra and how some people are asking why he didn't just leave her behind to die or kill her himself?
First things first - man, ouch, quite heartless there.
Second - as explained in the chapter, that would become a dark secret, how Indra is willing to kill teammates for his secrets, and we know the thing about secrets don't we?
They always come out at the most inopportune times so Indra doesn't want to have any such type secret.
Third - Indra isn't some heartless monster who can kill like breathing. His first kill took hours with him needing to justify it to himself to complete, he isn't at the level yet where killing is second nature as he has just become a shinobi and had a first normal life.
Fourth - And finally, why didn't he genjutsu memory wipe her?
Because Indra isn't that familiar yet with that, doesn't know how to really do it and if he tries could fry her brain and we'll have problem number 2 but worse since this is clearly the work of a Dojutsu and Indra is prime suspect of killing a teammate.
Well, I hope this cleared that up.
I hope to still see you all later,
Bye!