Wow. A lot of possibilities as to which whom Sasuke and Oro are referring to.
First, 'The human who knows everything' and 'them' could be two different things. It could be a translation thing, too.
It's not necessary that 'they' are among the living. And its a possibility that there is a jutsu with which one can travel among the dead and talk to them or have a conversation with them without Edo tensei-like consequences.
The human who knows everything sounds like a sage to me.
Kishi loves symmetry, as do all artists, so the legendary sennin (sages) were all capable of sage-mode. The next generation, as we've read countless times, always surpasses the previous. Therefore Kabuto's SM surpassed Oro's, Naruto's surpassed J-man's. Not looking forward to 'Hell-yeah' girl surpassing Tsunade... but, its probably coming.
I believe that 'The Human who knows Everything' is the So6P. I don't expect him to be Edo'd back to existence. The scroll will make it possible without prerequisites of ET. For a one-time conversation.
And, 'them' will be his parents. To ask 'Why the Coup D'etat?' 'Was it worth it?' 'Were you fooled or forced?' (by Danzo or Tobi or Madara) 'What were you all thinking?'
Big Questions:
1. If 'only Hashirama can defeat Madara' and Madara survived to live after VoTE battle with 'part' of Hashirama and achieved Rinnegan 'shortly before his death' (at some undetermined point in time after VoTE). Was Hashirama brought back to life by Tobirama using ET to defeat Madara the second time? If not, how could he have died?
2. If Madara 'made' Tobi by infusing Hashi-cells to a resurrected Izuna using that path of Rinnegan, how long were they in hiding? When did Akatsuki REALLY begin. I assume there existed an Akatsuki which utilized the Kin-Gin brothers, Zetsu and Kakuzu for example, lead by a masked Madara with 'Tobi' as one of his underlings.
3. How did Hashirama die?