Plot hole?

Just finished watching this movie today. Loved it and left me absolutely giddy since I'm such a big fan of the games and all. One thing that had me scratching my head was this though...

In the game, Von Karma wanted Edgeworth framed so he could exact total revenge on his father for spoiling his perfect winning streak.

In the movie however, is it not mentioned he won the case against Gregory Edgeworth anyway b/c Karma had killed Edgeworth Sr. before he had the chance of revealing Karma had tampered with the evidence? So by preventing Edgeworth from ruining his perfect streak, why go after Miles then? Miles had no suspicions towards Karma, and even believed his own father was in the wrong. Unless I'm missing something, this doesn't make sense to me.

Also, how exactly did Karma tamper with the evidence in his trial against Gregory Edgeworth? I know the gun he brings in, which eventually became the murder weapon towards Gregory, was the forged piece of evidence. However, how did he make the bullet's markings match the wrong gun then?