Under California law, the Medical Examiner is both required and empowered to determine the cause and circumstance of certain deaths (California Government Code Section 27491). After reading up on the Code, and just the "Investigative Report" (preface to AR), it became more clear to me why Dr. Lucas, the author of the AR, did not give evidence. IMO, Mr. Greer would be able to impeach Dr. Lucas as a witness by merely crossing him on the first 3 pages of the Investigative Report which begin:
Medical Examiner's jurisdiction invoked according to the California Code 27491: Death due to known or suspected homicide. The Narrative further confirms that both the Medical Examiner Investigator (Dana Gary) and Deputy Medical Examiner (Dr. Lucas) did not arrive at the scene until 7:15pm, and that RZ naked, bound, body was not bagged and transported to ME office until after 8pm! Oh, and did I mention that the 911 scenario in the report directly contradicts AS testimony...
I'm just speculating but I believe had Dr. Lucas testified, and impeached by Mr. Greer during cross examination, Lucas would not have been dismissed (he's credible and authored the report) but rather the Judge would have to instruct the Jurors to disregard his testimony. Just imagine the Defense being told that the court will not credit the Medical Examiner witnesses testimony. There really was no way they could allow Dr. Lucas to take the stand. (It's called the falsus in uno charge).
https://www.sandiegocounty.gov/content/sdc/me/families/theprocess.html