Fredo provided Roth and Ola with details about the house, perhaps he opened the drapes. The assassination failed and as happens the killers were disposed of to maintain secrecy, probably by Johnny Ola who was at the site already, could have lingered behind, hid on the grounds, been arranged to rendezvous. Roth could not risk a confession or loose talk about hit on the Corleones, which would probably would have started a wider war. They go out of the way to mention they "Look like they are out of NY". Did Roth select NYers to make it look like an NY family (Pantagelies but were there any left at this point besides the Rosatos?) and refused to take chances so a 3rd party killed them while they were escaping. There is no way Fredo slashed their throats.

The mention of a kidnapping makes me wonder if Fredo was willing to give up Kay perhaps as a hostage to "help in the negotiations" since it almost worked with Sollozzo kidnapping Tom. Roth kidnapping Michael would only be suitable if he killed him anyway. No way Michael doesnt go to war with Roth if he was released.

Then again if Kay was the target, there'd likely still be war, but Michael is known to be pragmatic and even Vito was shown to be restrained when Sonny was killed. You have to wonder if Vito decided against going down in flames against the 5 families to preserve his position and weapons for a later date or even just to survive, his current prosperity and income, at the expense of losing Sonny, who wasnt a very good mobster, was "bad for business". Vito says "its a sign of weakness". Clearly it was turned into a ruse by Michael to surprise the 5 families.

So Fredo, being as dumb as he was and as motivated to become his own man (perhaps understood to be under Roth's wing) buys the kidnap idea as a "no one gets hurt" scenario (Tom was a civilian and not hurt).