Do you know if the toxicology report includes info about other substances, like PCP, hydrocodone, Robotussin, etc.? If they failed to test him for something like maybe the ingredients in purple drank, can the defense bring that up?
Positive for pot, but other substances either weren't tested for, or the results have not been made public.
The report is probably available somewhere on the internet, but I haven't seen it yet.
As for your last question, It would be my opinion that they can't bring up something that wasn't tested for. To ask 'why' it wasn't tested for, the defense would likely get shut down by the judge. Legally and reasonably.
Look at it this way. We can't go back in time and do something we now think should have been done. Ergo, the opportunity was lost, and there is no solution to arguing about why.
Again, it's all dependent on what is actually in the TOX report anyway. Maybe it shows more than we think, or know about.