I guess the Cartwright and Chang factions knew (or thought they knew) all there was to know about the severity and significance of Praxis, whereas official Starfleet had to wait for negotiations with Gorkon before they could send an assessment team.
It sounds likely that Cartwright knew of Chang's involvement and vice versa, but that both of these old enemies thought they'd get the upper hand from the deal. Chang probably explained that he just wanted to get rid of the old fool who was ruining his Empire, and would leave it at that; Cartwright knew Chang would be more of a threat to Starfleet than Gorkon, and liked it that way because he thought Chang would go to a war that would be the ruination of the Klingon Empire.
Cartwright in turn would have explained that he'd supply Chang with Federation torps and other assistance because Gorkon's peace initiatives would cost him his cushy Starfleet job, and if Chang also eliminated that annoying popstar Kirk during the assassination, Cartwright would happily assist and leave it at that. Chang agreed and thought that inserting a little bit of ambiguity to the guilt issue would be helpful in the upcoming inevitable war, perhaps slowing down Starfleet response the necessary little bit that would allow the Empire to triumph. Plus, it would always help to have Kirk out of the way.
And Nanclus told both, separately, that Romulus would assist one side and not the other...
Chang had a time limit on getting his much-needed war, because his Empire was weakening. Cartwright would probably have gained from stalling, but the Khitomer Conference threatened to create a peaceful outcome, so he hastened things by trying to frame the Klingons for another assassination. And Romulans seemed to be central to organizing Khitomer, probably with the explicit purpose of forcing Cartwright's hand and stopping him from stalling, so that maximum destructiveness to both sides could be guaranteed in the upcoming war.
It's not really all that important whether Praxis blew by accident or by design, but the opportunity and means for Romulan sabotage would be there.
Timo Saloniemi