I understand where you're coming from and you're right when you say the von Karma would never have settled for that, he instead would have forged his own evidence to prove it. But Phoenix himself had no evidence to prove that he wasn't the one who forged the diary page. The second the diary page was proven to be a forgery Nick was considered the guilty party. He was the one who presented it to the court and he had no proof besides his own words that he hadn't ordered it. Phoenix had a hearing after the trial where he would have had the chance to plea his case, but because he had no actual evedince otherwise everyone on the hearing comitee beside Kristoph declares him guilty. That's how the court system works in the Ace Attorney world, all suspects are guilty until proven otherwise.
The Steel Samurai case is a good example of this, Will Powers had no motive and it was proven on the second day of the trial that he couldn't have done it. But he was still considered guilty until Phoenix proved someone else had done it. Stupid as it is, Klavier didn't need any proof beyond proving that the diary page was indeed a forgery. It follows the logic of the previous games.
The Steel Samurai case is a good example of this, Will Powers had no motive and it was proven on the second day of the trial that he couldn't have done it. But he was still considered guilty until Phoenix proved someone else had done it. Stupid as it is, Klavier didn't need any proof beyond proving that the diary page was indeed a forgery. It follows the logic of the previous games.
