Such -the misdoings of all other police forces from then until present day- seems to be the only reason you can put forward for Jeremy's innocence.
Yeah, it doesn't really work does it. The police must have been corrupt in this case because there are other cases where other police officers have been?

Why not say that JB must be the murderer because other people have committed murder before.

I'm all up for the possibility of such misdoings, but I'd need to see evidence of that being the case and there simply isn't any, or at least none which has come to light.
I think if people want to be taken seriously then they should be a little more responsible with their accusations.