If it can be proven beyond a reasonable doubt, they should be prosecuted.
If it can be proven by a preponderance standard, the prosecutor should issue a Brady notice to the defense bar, which will in most cases naturally result in the police officer being fired or assigned to administrative duties.
I respect and appreciate your position. I believe we both have high expectations of prosecutors and law enforcement.
At the risk of presuming too much, your position wrt QI and other reforms might be summarized as "We already have sufficient rules. The rules must be followed. The proposed reforms would eliminate an important safe guards as well as not achieve the desired outcome."