Just because the pills were not found in her stomach doesn't mean they weren't in her bloodstream.
He should have left the result as "undetermined" because he cannot know "accidental" anymore than he can "know" homicide.
Remember, it is an opinion.
However, he should have been more diligent in laying Danny's death drugs side by side with Anna's and brought out WHO prescribed for Danny because as of this date WE STILL DON'T KNOW who Danny was seeing for depression or prescribed his drugs.