Welcome BR,

It's always good to resurface an old question. It gets the juices flowing and makes us go home, pop in the DVD and run through the scene again to come up with some crazy theory.

Theory # 537. If you recall the assasins (who look like they are out of NY) were found in a small stream / drainage ditch. There is a large drainage pipe there. They probably snuck onto the grounds through that pipe. There were three of them; two button men and one Luca Brasi-type who was associated with the Rosatos. The button men did the shooting, ran back to the pipe and got killed by Luca Rosato. Luca escaped back out the pipe and hopped into his waiting boat and sped across the Lake before security could button down the place.

The drapes were opened by Fredo. The ruse was this, Hyman told Johnny O to get Fredo to open the drapes because they were planning to take pictures of Kay in her nightgown to use as a little joke / surprise for Michael's birthday.