Well, this is the Great Mystery of the Boards, given how often the topic has come up. We'll never know for sure, and that's part of the fun of being a GF Trilogy nut. I'll offer a few guesses :
--Fredo probably knew it was gonna be a hit, and may have helped by opening the bedroom curtains so that the shooters would have a better shot. Yeah, he's weak and stupid, and is "so sweet," seems like an unlikely fraticide. But note the venon he hurls against Michael in the boathouse scene: the mark of a guy with a serious grudge.
--The dead shooters may have been with the Rosato Brothers. They're not recognized (so they can't be Johnny Ola's men), and they're "out of New York," which is where the Rosatos operated. Roth may have convinced the Rosatos, with whom he was allied, to supply the shooters. Then Roth probably arranged for the shooters to be killed.
--Who shot the shooters? No idea. Notice that, after the alarm goes off, you briefly see gates being closed and a car screeching away. That car could have been a Corleone security car about to patrol the estate, or perhaps the shooters escaping. I doubt that they were Johnny Ola's men: too risky to leave them behind for that job.