The difficulty with introducing expert witnesses to address credibility issues is that in some global sense the defense ends up looking like they are asking the classic guilty man's question: "who are you going to believe, me or your lying eyes (ears)?
And expert at a trial usually is met by another expert with an opposing chart/ theory explanation etc. Yes, the defense can go down that path but I'm unsure where it would lead except to the prosecution asserting a different and much simpler explanation for the contradictions and inconsistencies which they are sure to do anyway only in thiscase with a guy in a white lab coat asserting the same thing, basically.
GZ already tried the fear, mistrust and confusion defense with the judge regarding hidden money and hidden passport issues. It didn't seem to play well, IMO. Attempting to use ADD or ADHD and or PTSD to excuse or explain inconsistent statements to police is certainly an option but bear in mind the officers can and will be called to the stand to give a much simpler explanation for seemingly contradictory statements that are met with equivocation on the day and "experts" in court. Also said experts will be asked on cross "were you there that night" and, "is another explanation possible, to wit, that GZ is /was simply lying?".
As for GZ taking the stand, yes he has that option but then faces cross. You tell me what inconsistencies and contradictions he might be asked about and how his earlier " explanations" might effectively counter them. You may not be a lawyer but you are at least a juror for the sake of argument.