Okay so everything runs the same until Mondo kills Chihiro. Taka is looking for Mondo because he couldn't find him and he walks it in the immediate aftermath of the murder. Mondo is preparing himself mentally to be executed but Taka goes "We can fix this".

So it goes like it does in the game but Taka divides the jobs to conceal the crime between them. Sends Mondo to wash the blood off himself and Taka alters the crime scene. Then they try to act natural.

It doesn't work. They have the ultimate detective in their class but Taka has a trump card. When Mondo is discovered he drops the bomb that Mondo didn't kill Chihiro. It's not a bluff and Taka is such a bad liar everyone knows.

While Mondo was washing off Taka realised that monokumo didn't announce the body had been discovered when he came in earlier. So he checks and Chihiro is hanging on. He's clearly not going to recover so Taka decides to put him out of his misery. This has the bonus of protecting Mondo by making himself the blackened.

Mondo is crashing out but Taka is calm. I don't think he ever expected they'd win this so he's been preparing for this moment. Mondo vaulting over his little witness stand to take the most direct path to Taka. Begging Taka to tell them he's lying, screaming shouting. The only reason he doesn't get himself killed right there by attacking Monokumo is because Taka holds him back.

Taka begging Mondo to keep his promise to Diaya and making Mondo promise to live for him. Giving Mondo his room key and it's too much even for his composure and he starts crying.

Monokumo dragging Taka away for punishment and his moral enforcement committee band gets pulled off by Mondo trying to stop Taka getting taken away. Parallels with Mondo begging his classmates and Monokumo to not execute Taka "He wouldn't have even done it if it wasn't for me!"

I've got 2 ideas for Taka's execution both based on the same base. His unused one is Kiyotaka Ishimaru's inaugural parade and it's based a lot of that. He's strapped down forced to be paraded around but here's the split.

Idea 1: Mondo is unable to control himself, breaking into the execution area and trying to free Taka. Taka desperately begging Mondo not to do it to leave before monokumo punishes him. He manages to save Taka from the first shot but just as they think they're safe monokumo pops up behind them and shoots Taka and Mondo is just sprayed with blood. Mondo holding his lifeless body in a direct Daiya parallel.

Idea 2: really emphasize that it's not just Taka's punishment but Mondo's too by having Mondo also tied down with Taka. He's sitting next to him in the parade like the prime minister's partner does. Helpless to do anything but watch Taka die and still get sprayed with blood. And Taka dies scared, he dies thinking that he didn't change anything and Mondo is going to die.

Either way it destroys Mondo and he tries to goad Monokumo into killing him too but the whole point is leaving him alive as a worse punishment. Mondo shutting down because he can't process his emotions and physically locking himself in Taka's room.

Mondo only comes out to see alter ego because he feels responsible for both Taka and Chihiro's deaths and maybe he can apologise to alter ego instead. And then he Kiyondo!Mondo's out after alter ego turns into Taka. Kiyondo!Mondo finally being able to process his emotions and cry about Taka's death (and Diaya to a degree) rather than holding everything in. Learning to let himself feel actually helping Mondo be less angry so he doesn't get into as much conflict with others.