I agree, Turnbull.

The idea is to not let your enemies know what you are thinking....

In this, Fredo seems innocent to make it appear that he is. Remember in the novel, after Santino is killed, and Tom calls Carlos....he reassures him that noone thinks that he killed his brother in law. Most importantly, Carlos admits that his beating of his pregnant wife was not an effort to draw out Sonny. He wanted it to appear of his terrible relations with his wife to make the hit successful.

I am saying all this to say, that Fredo is not innocent in all of this. Maybe he did not think it was going to be a hit, but certainly, he was one of the plotters.

As far as who killed the hitmen, is a matter of much speculation that can never truly be answered. (MP should have developed this line of action in the screenplay!)