...instead of Celestia getting Kiyotaka & Hifumi killed and framing Yasuhiro, she instead killed Yasuhiro and framed Kiyotaka & Hifumi? I've always felt like if one really wanted to best show off the Ultimate Gambler's skill, it would be through her forcing the rest of the current surviving class into what she considers the Ultimate Gamble.
Consider this, what if Celestia tricks Yasuhiro into a situation that allows her to kill him and then she plants two sets of false evidence, one against Hifumi and the other against Kiyotaka. Both sets are arranged to make it seem like Hifumi or Kiyotaka are the killer trying to frame the other while falsely planting evidence against themself to sell the narrative that they are the one being framed. Given their already on-going rivalry over Alter Ego, it'd further enrage them both into going for each other's throats.
Come the trial, you then have both Kiyotaka and Hifumi acting as major distractions and roadblocks during the discussions, each trying to steer the trial in their favor against the other, all while Celestia sits quietly at the sidelines, acting all the innocent girl she claims to be. As the case goes on, it seems as though the other students will be forced to gamble with their lives on which of the two boys is the culprit, only for Makoto to snag onto the hidden ace that unravels the scheme and sets him on the path to pin Celestia as the true culprit.
Not only does this allow a potentially better case that lets Celestia use her talent to set up what seems like a life or death decision that's secretly a death or death, but it also lets both Kiyotaka and Hifumi live on to develop and perhaps even go on to survive in Yasuhiro's steed. You can have Kiyotaka come to terms with how unhealthy Kiyondo's mindset was and live on for Mondo instead of trying to be too much like him while getting to mourn his lost friend properly, and you can have Hifumi learn to try to make more of an effort to connect to his remaining classmates around him, instead of limiting himself to people like Celestia who'd use him or allowing his interest in something like Alter Ego to get the better of him.