I'm afraid the way you describe it is nonsense, I have heard of this untruth before, it is the "hell hath no fury like a woman scorned" myth.
She didn't go to the police because she had been dumped by bamber, their relationship had been going that way some time and he had more or less completely ignored her anyway since the funeral.
She went to the police as her conscience unravelled when the realization hit home of what he had done, and that there was no 3rd party involved.
Happy to assist with any questions you may have about the case.
I obviously don't understand you. We know factually
1. Mugford identified the bodies soon after the shooting.
2. She can imagine Bamber, Matthew MacDonald or Sheila as being responsible.
3. It can't be Sheila because Bamber has said he is organising the massacre, he phoned her earlier and said it must be tonight, and afterwards phoned her at 3 30 saying it's all going well.
4. By the time she is in court she claims it was Bamber.
So any dawning realisation must be that it wasn't MacDonald, it was Bamber. Either way she knows Bamber paid someone to put 8 bullets in the heads of the sleeping 6 year olds, or Bamber put 8 bullets in the heads of sleeping 6 year olds.
Somewhere along the way she is lying, and the only lie that fits is that he told her he was planning it.
Since we all know that she would not plan holidays with a child killer, she most assuredly did not know he had organised it, so she lied in court when claiming foreknowledge he had. She was a groomed witness.