Fabrication of police statement

Why did the defense never question the fabrication of the police statement (or even mention its existence in court)?

The statement has clearly been altered in order to provide a confession that Anthony Nealy did carry out a shooting (we have copies on record).

However, even if Anthony Nealy had admitted to being involved in an accidental shooting as the second and later part of the 'statement' suggests, why did the Defense not press for the jury to be provided with the option of a verdict of 'involuntary manslaughter'?