Ola asking Fredo to open the drapes is total speculation. Nothing even close to that happens in the film. Anyone could have done it.
Nowhere in the film is it shown that Fredo opened the drapes. But, as we saw, Kay was surprised that the drapes were opened, and the shooters fired at Michael through the window. My conclusion is that, with thousands of people on Michael's estate for Anthony's party, Rocco would have assured that guards were posted around Michael's home to prevent people wandering in to take a look inside, or perhaps to lift a "souvenir." But Fredo would have been allowed in. And Fredo had motivation because he was cooperating with Ola against his brother.
As I've posted elsewhere, I don't believe he was promised anything in particular. He was so eager for something on his own that he jumped at vague promises from Roth and Ola.
I also believe that Ola/Roth didn't promose Fredo the Donship or anything specific. They didn't have to. We saw the depth of Fredo's resentment of Michael in the boathouse scene. Ola must have picked up a lot of that when he "bumped into" Fredo in Beverly Hills--probably over drinks.
Logically:
--Fredo seethed with resentment against Michael because he thought he should have been the Don. He said so: "I'm your older brother, Mike. And I was passed over/"
--There was only one way for Fredo to become the Don-: if his brother were no longer the Don.
--The central event in the attempt to kill Michael was opening the drapes so the attackers could have a clear field of fire on Michael.
--Fredo was the most likely person to be in a position to open the drapes, and was asked to do so by Ola.
Though Fredo may not have wanted to actively think about the consequences of his action, fhere was no logical--possible--nonviolent consequence of his opening the drapes. When he said to Michael, "I swear to God I didn't know it was gonna be a hit," I think he meant that Ola didn't explicitly tell him it was gonna be a hit--and Fredo conveniently didn't ask.
