The way I figure, there are 2 things about that scene that don't make sense: First of all, where was Fredo going during the New Year's celebration when Mike asks him and he replies, "I'm going to get a real drink, cause I can't..."? And second, where did the military guys come from? It seems to me, the simplest explanation is that Fredo alerted them. He knew about the hit. When he and Mike are enjoying banana daiquiris, Mike tells him that Roth won't live to see the new year. Fredo probably figured that with Roth dead, he would never get whatever was in it for him, on his own, so he decided to foil Mike's assassination attempt.

And you're right, Bruscetta is a resourceful fella, but my point is that if he would have used a knife or something instead of a pillow, Roth would have been dead.