edited by me for brevity.
On March 8th, VICPOL arrested Stephenson for murder, after consultation with the Dept. of Public Prosecution , who , as their label implies, takes VICPOL's cases to court. The police hand over all their stuff to the DPP and the DPP decides if a case is valid, and winnable. In the DPP are a cohort of very astute and learned Barristers, Lawyers, Paralegals, etc, for whom this is their profession. It's what they do, all day, every day, go over evidence with a fine tooth comb and after consultation, make decisions in regard to the progress or non progress of a matter bought before them.
So the DPP would be not in agreement with your conclusion at this point, and since they have maintained this position, that Stephenson murdered Ms Murphy, with intent, and because it is murder, with malice aforethought, and they claim they have the evidence to back this conclusion up, it is highly unlikely that they could be persuaded by anything to downgrade the charge to Manslaughter.
The defence has had all the evidence pertaining to the case the DPP intends to bring to court, quite an unprecedented amount ,and the Prosecution has the list and particulars of all the witnesses, (8) that Stephenson intends to bring to testify, PLUS (1) extra who's statement has not quite been finalised, when this is done , it will be handed to the prosecution, under disclosure orders. The defence itself has not argued for the charge to be downgraded, at any point in the proceedings . And they have had 3 hearings so far to propose this change.
( As to the mystery 9th witness, I wonder if it is a psychiatrist who has been treating Stephenson all this time.. very late witnesses are rare, what would be the reason for someone holding onto information all this time, if it is not some medical matter ? .. I can think of only one very late witness and that witness was for the prosecution, in the case of Robert Farquarson's second trial, she had been ill, she had moved to NZ, but she caught up with things when she returned to AU, her testimony was devastating to Farquarson's story, it absolutely buried it. He was convicted of murder of his 3 small sons. )