As far as cops not investigating cops...than who? I hear this all the time but no one has an answer. Law enforcement are trained as professional criminal investigators. You can't just find someone on the street to conduct a potential criminal investigation when they have no experience doing the multitude of things that need to be done when conducting a potential criminal investigation (obtaining search warrants and subpoenas and executing them, forensics, crime scene analysis, etc...and then turning over that evidence to a DA or AG for prosecution). If you want the Feds to do it, fine...they are still law enforcement and it won't satisfy those who think law enforcement protects law enforcement.
A group of individuals that are paid by a completely separate entity would be a good start.
Collusion is a big concern in these situations. Representatives from random counties and perhaps a mix of individuals with backgrounds outside of police work.
Just throwing out ideas here, so feel free to shoot holes through it as a first reaction. hahahahaha