This is something we studied at law school - but that was back in the 90s for me so this is some broad strokes
Alternative counts
Manslaughter is actually already included as an alternative to murder, and where it emerges as a serious verdict option, the judge will so direct the jury - even if it was not charged in the indictment.
Homicide: Murder and Manslaughter | The Crown Prosecution Service
However it is generally preferable that the Crown would charge manslaughter in the alternative to murder where it appears as a serious issue at trial. This might be where the accused intends to argue a partial defence. So e.g the accused admits killing the victim but claims it was merely negligent, or where a jury might infer negligence - Pistorius is an example of a case where the judge did not find murder, but found manslaughter as an alternativeat trial - despite the Crown arguing for murder.
All that said, there are tactical reasons why the Crown might not include manslaughter in the indictment
In the present case, the the Crown declines to do so, because their case is murder and never manslaughter. Critically this prevents PR making a guilty plea to manslaughter.
In the present case, neither crown nor defence alleges manslaughter - so in my view the issue will not arise.