Heh, sorry, I'm two months late to this, don't mind me...
You're quite right about the forced confession. It wouldn't fly in a courtroom, and the same goes for the LINE chat printout. But those rules wouldn't apply here, just like boxing rules don't matter in a street brawl. These underhanded methods were only used to get Niina into a situation where her phone's contents were in focus... and she took the bait. Nobody in that room was going to ignore what they'd seen, and there wasn't any judge to tell them so.