The only thing I'd add for consideration here is that it's not really the prosecution's job to worry about a defendant's mental state; as I understand it, it's up to the defense team to bring that sort of issue to the attention of the court (ie the judge). Not to do so, but then to bring the matter up in negotiation, looks an awful lot like an attempt at manipulation instead of a sincere concern. The whole idea of the court as a neutral finder-of-fact is to provide defendants with an unbiased ear for such issues so that they're not dependent on the good will of prosecutors.
Courts are messy affairs; there's still a lot we don't know, like the disposition of the judge, so I can accept that the defense should have brought that to the court, but would rebut that we don't know that they didn't, or that they weren't able to for some reason. Either way, I don't think that completely exonerates the prosecutor's office in this case.
Thanks for the calm, reasoned discussion.