After a few minutes of thought, I think that the strength of the evidence might have an effect on how vigorously the prosecution fights the defense position that she was acting in the role of a police officer.
Why ? Because not to allow it under these circumstances (in full uniform in transit from work), could be reversible error.
If the prosecution thinks that the evidence is too strong for a rational jury to find her actions “reasonable”, then the prosecutors might allow (not object) such a defense in order to protect police officer actions in future situations when in full uniform. (After all, prosecutors need police officers as witnesses in most criminal cases.)
Also, it is difficult to define “coming to the aid of another person” or “in aid of another” or “in protection of others”. A police officer often only has a split second in which to act. The easiest analysis is : “A crime is a crime is a crime” whether against an officer or a civilian.
Nevertheless, even if acting in the role of a police officer, this appears to be a murder–not just manslaughter–based on the publicly available information. Hard to imagine any reasonable reasonableness defense at this point.