True enough, there are little ambiguities left in that will remain unsolved because Michael killed Fredo and didn't ask him.

Let me just put it this way, if the windows were not open then Michael wouldnt have caught a glimpse of the men outside and wouldnt' have reacted as quickly and could have been killed easier.

In the GF Extras DVD FFC elaborates by saying he enjoyed adding a little twist or imagry in a violent scene that would make it memorable and distinct such as the towel wraped around the gun used by Vito to kill Fanucci or the clothes hanger used to kill Ola or Vincent shooting the last assailent through the hand when he killed him after the failed assasination attempt.

I will conceed that this is a true loose end in the movie, but I think this fact is weighed out by Fredo's own admission shortly after in the late night phone call from Ola that he wants nothing more to do with them and they didn't say it would be a hit, they indicated it was just to talk to him. So I beleive this outweighs any curtain that may have been left open by anyone.