I think, the Tattaglias didn't really suspect Luca of being an infiltrator. It was Sollozzo who saw his chance of killing Luca, no matter what Luca's plan was.
The weakness (or slippery) of Vito was that he didn't foresee the power of the Sollozzo/Barzini/Tattaglia coalition.
Turning of Paulie, the hit against Vito AND Luca and the abduction of Tom were very well planned. It was simply bad luck for Sollozzo/Barzini that Vito survived. And it was good luck for Neri/Michael that Barzini died. (Even in the novel version of the killing, Barzini's bodyguards just froze, unable to help their boss, just like Fredo.)