I got the impression that the emails were defining answers to questions posed by West in an effort to get better acquainted with the material he would be confronted with during the Frye hearing. I never really thought that it would be an opinion.
They could be "are you available," "are you qualified," "what are your qualifications," sort of intro stuff, as well as apprising the experts of the case. Additional input to the expert/consultant (there is a distinction) is generally evidence, maybe opinions or reports of opposing experts. Good lawyers don't lead experts. The experts are led by the evidence.
Then, along the lines you mention, the consultant may suggest weaknesses in the reports of opposing experts (this is clearly protected attorney work product) by way of assertions or questions about standards, methodologies, assumptions made by opposing experts, external evidence not taken into account (sounds by W6, televisions in the room where the 911 caller is and so forth) and so forth - substantive in a general sense, but not stating any particular conclusion based on the evidence in this case. Nelson would be looking for remarks where the expert independently evaluated the evidence with an eye on forming an independent opinion. Absent remarks along those lines, there is nothing in the nature of "report and/or conclusions," and that is the material that is discoverable under the discovery rule, 3.220.
In the realm of expert testimony, the defense is obliged to produce ...
reports or statements of experts made in connection with the particular case, including results of physical or mental examinations and of scientific tests, experiments, or comparisons
As to the distinction between consultant and expert, the consultant can analyze the evidence, and might (being independent) reach a conclusion that is unfavorable to the defense. This person would not be called to testify, but might be very good at poking holes in framing of the case, the methodology, and the exact conclusions reached by the state's experts, even though the consultant might have an opinion that comports with the state's case. That is in the nature of attorney work product, and is not discoverable.
Any time I am asked to review evidence, I insist on being taken on as a consultant, because the only things that lead my thought process are the evidence in the case, and scientific, engineering and technical materials. It is common for me to form a preliminary opinion that is not favorable to the side that contacts me.