Would the police really want to pursue charges against Holmes?
They never like to pursue charges against official members who resort to torture and other forms of violence against perpetrators. Sadly it is highly unlikely they would want to make a case against an unofficial member. The unlikelihood of a conviction really makes it very improbable, were such a thing to somehow occur in reality.
Well, as Bell himself said last night, it would be easier for the NYPD to deal with Holmes if they paid him -- because then they could fire him.
He's not an official member of the police. He's an unofficial consultant.
And the real problem with charging Holmes with a crime is that Holmes would reflect badly on Gregson. He's let Holmes in on a number of investigations -- including the Moran affair -- and he would have to answer for why his unofficial consultant kidnapped, attempted to torture, and assaulted the subject the suspect.
For Gregson's own self-preservation, he needs to maintain the fiction that Moran got the drop on Holmes and Holmes attacked him in self-defense. That doesn't mean that he has to like it. And he clearly doesn't. But there's nothing he can really do.