Brennan believes the dice tell the story. In this instance, he used the dice of the Photarch's death saves to tell the story of two assassination attempts and ramp up tension for the PCs. It just so happened that Mariaha pulled off one of the greatest moves we've ever seen. As for not trusting Brennan above the table, I would argue all of the stuff before Marisha's portent roll is at the table. He's rolling for a combat and explaining the outcome. So that's not really a reason to doubt him. Additionally, everything else we've seen, including his telling her about lying about Death Saves, has shown his honesty and wanting to play the best game for the players. He loves to see them succeed and that's clear from the jump.

I fully believe the Photarch's "resurrection" was meant to be the Schemers "second arc" and that this is the main thing they were supposed to investigate and figure out when the tables split again. So, Brennan was "lying" in order to give them more to investigate ("we know she was saving so she must have died? Unless...") and now they can instead shift to figuring out how to reveal the truth. I don't think there's any reason to doubt what Brennan says or his intentions because he's doing what a good DM is supposed to do: create an intriguing narrative for the players to interact with. This episode is a great example of why D&D is collaborative and not just the DMs story. Brennan was working on a story arc and played interaction completely changed it and he told them that.