Mate, let's pull it back a bit, and look at it objectively and rationally. Looking at it literally, you're entirely correct about the point you've made here.
They could, and with hindsight
probably should, have contacted the police. Yet, emphasis on should here as we need to understand what their concerns were.
I don't know the case in enough detail to definitively say x, or y, so it's a bit of prediction based on what I have read of it, and other experiences etc.
First I'd ask: were their concerns suggesting her actions were illegal (e.g. did they meet a threshold for a crime) or were their concerns of negligence?
If it's the latter, then are we talking about the negligence being to the extent that it is in fact criminal, or are we talking about unintentional poor practice by her?
This is where an investigatory department should have come into play. We also have to consider the expected procedures that most organisations have.
In the simplest sense, we've probably all seen the flow charts: report to this, they go to this; not dealt with, report higher up the chain. You catch my drift.
In most cases, rightly or wrongly, it is frowned upon to come away from the policies in place, and I know of examples where pressure has been exerted.
Now, I am not excusing people here because it reached a point where, admittedly, I'm flabbergasted how it didn't a) get reported earlier, b) become public.
But, an independent investigatory department (e.g. professional standards), who are detached from the executives, could have easily picked this up.
From what I can tell, there were numerous examples of where people, likely for their own benefit, swept her criminal actions under the carpet.
Others may have done it unintentionally - protecting the reputation of the Trust or not having the knowledge to investigate - but they're still culpable.
Whether you agree with
@COYBL25's recommendation that detectives sit in it or not is irrelevant to me, but people with investigatory experience would help.
People have obviously whistleblown about her, to the point numerous consultants have a wrote letter pleading for her removal, and they end up apologising to her!