--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.
The drapes being open is a central event because we actually see that and because Fredo confesses to something and opening the drapes is about the only part of the assassination that we can imagine Fredo pulling off. Admitting or concealing the shooters, the shooting itself, and murdering the shooters are also pretty important events.
The only non-violent speculation I've ever come up with for why Fredo would open the drapes is that Roth's people would want to search Michael's office. So they had to know when Michael was turning in for the night. I think that works all right if it's two sentences on a page. I think it fails, though, as dialogue between "it'd be good for the family" and "You believed that story?" So, for me, that's where Coppola and Puzo just stop - with a strong emotional story, but with the details intentionally left ambiguous.
