Underboss,
You and I both know that we don't know who left drapes open or if the hitman had gotten a lucky break and took advantage of opened curtians. So trying to discuss that is a mute point.

Fredo was not thinking about the family's welefare, rather by his own admission to Michael (boathouse scene), he wanted something on his own. He despised being 'kept' by his own brother. For this reason he sided against the family. Plus, there is two indications that he wanted to keep his relationship with Ola/Roth a secret. 1) the phone conversation with Ola: Ola tries to assure Fredo that Michael will never find out that they talked. 2) Fredo denying that he knows Ola/Roth to Michael in Havana.
Underboss, if Fredo is moving on the better interest of the Family, what gives with all the secrecy?