Re: What is in Oro's Scroll [Theory]
Your conclusion that it must be edo tensei is bad. It could still be other things, so we can't just neglect other possibilities.
What we do know:
It's a human being
It's at a place Sasuke knows well
What Sasuke says, is, "there are certain things, only Orochimaru can do". He mentions no specific jutsu, so it could be other things.
Sasuke didn't know that they needed to go to a special place to do it.
Orochimarus job in this, is to make sure Sasuke can meet some human, who knows everything. Possibly more than 1, but that may be mistranslated.
The person will tell Sasuke something about, what Itachi went through. Itachi gave up everything for his brother and his village.
Uchiha Madara - We still don't know his true purposes. So far Kishi has painted an image of a power hungry animal that just wants to cause destruction. But this doesn't make sense. Why did he stay out of Konohas affairs after Hashiramas death? He could have used perfect susanoo, and erased the village from far out. Something about Madara still isn't clear, but we can only guess, what it is.
How does this implement Oro? Perhaps Oro knows a way to resurrect Madara, or move his soul to another body. What would then happen? A lot of hints have been given that Sasuke will fight Madara, so this would be the perfect opportunity. They could be heading to the akatsuki hide out.
He knows, what Itachi went through regarding the village. His body was never found. Oro could have it in one of his lairs.
Resembles Sasuke a lot. What purpose did he give his eyes to his older brother for? What would he, have wanted his brother to use his eyes for? What kind of future did he want for the Uchiha clan? What would he think, of what Madara is doing with his eyes now? His remains could be in the uchiha hideout. Oro could use edo tensei. Izuna could make Sasuke want to take down Madara.
I don't see other possibilities actually.