I think you are on to something Apple.

I think that Deanna was somehow involved with Merle and she was the distraction at the party (all drunk and dancing) so that Merle could slip in and open the drapes.

See, Mama Corleone knew there was something about Merle and Deanna.

As for Fredo getting Anthony to help...it's a good theory, and I could see Fredo being stupid enough to use an accomplice...but I got my eye on the Apple theory