I agree that McCulloch potentially should have recused himself due to conflict of interest. However, the forensic evidence was collected independently of McCulloch and speaks volumes about what actually happened. The pro-Brown stories simply do not fit the data, and virtually all "eye witnesses" who previously defended Brown have retracted or changed their statements when testifying to the grand jury.
That said, there is an independent FBI investigation going on, and since there's no such thing as double jeopardy for grand juries, we may yet see Wilson taken to court. I doubt we will - I firmly believe in the forensic evidence and the testimonies already given - but if the original investigation fucked things up, these errors will be corrected. Importantly, there is also a second FBI investigation looking at Ferguson at large.