I've been re-watching some of the earlier episodes to see if I can pull some of the still slightly mysterious (at least to me) strands together.
In episode 3 (I think), it's made clear that there is no love lost between DI Williams and DCI Parry. So at that stage at least, they weren't in cahoots as far as illicit or illegal goings-on were concerned.
Williams is dishonest, but maybe only in the sense of doing anything to convict people she "knows" to be guilty. Of course in real life, we know of miscarriages of justice which have happened that way.
It would appear that Parry and DS Morgan are financially corrupt, although it's possible that the money was only part of a "sting" type operation (that had probably gone wildly out of control), and they wanted it back to try to cover their tracks. Anyway, by the end, Williams appears to be in cahoots with them, but I presume that's only in return for them letting her off the hook for the Alpay car crash.
Talking of Alpay, knowing now that Cerys has eclectic tastes, I'm wondering if she and Alpay were an "item" at one stage. It seems to be hinted at in ep. 3 or 4.
And I was reminded of the deal (which we saw in flashback) between Evan and Glynn. If we take it at face value then given that he must know what sort of a man Glynn is, it would mean he had become, or was about to become, a "bent brief". However, it's also (just) possible that he was doing it as some sort of undercover operation, to find out more about Glynn's business, and maybe help put him out of business altogether (working with Parry....?). Anyway, again it seems to have got out of control.