It's very weird that Cassis knows about the Agriche's poisonous flower research - could this have something to do with his memory of part of him being "locked away"? Like his power is connected to him knowing about stuff that no one else should know? Lol maybe Cassis's power is omniscience.
Or maybe his sister is also a reincarnator with knowledge of the novel, so she told Cassis about the flowers that the Agriche family created? I don't think she would've been able to stop the Agriche family from capturing him, even with the novel's knowledge, and she would probably think that in order to get her happy ending, she'd have to have her brother get kidnapped and tortured for the sake of plot.