It sounds like an incredibly distressing situation, and for me the most telling bit is that your DH now wants you to stop talking about it and 'move on'. How convenient.
I feel as if you'd be better spending the money with private investigator, if nothing else to help you sort through the categories of evidence you have and how reliable that evidence is.
1 The police have been involved because this woman's husband was convinced she was having an affair with someone at work, but suspected both your DH and another man. He put a tracker on your DH's car and followed him to your child's school.
I think this is your 'top tier' of evidence.
2 The woman in question told you on the phone she had been having an affair with your husband. But she seems to have MH issues, was living with an obsessive who was stalking people, and both her and her husband's stories kept changing.
I think most of the evidence you have about what happened in the workplace, as opposed to the stalking, comes from this?
Either the husband or she sent you screenshots of emails, not the emails themselves. So you had no way to verify if the header information had been changed (unlikely on workplace email that the end user can change the display name themselves, but the screenshots could have been doctored).
3 You also saw screenshots of things in your DH's handwriting, which he claims to have been writing in the office (bizarre) in his own notebook, and which she then stole. These were presented as attachments to emails, but you don't have any of the actual emails. He seems to deny having written out the song lyrics of one of his old songs and claims they got it off YouTube but how did it end up in his handwriting in that case?
So the notes are real but the proof of them having been sent to her is more flimsy. However, enter:
4 Other work colleagues - you mention someone else has told you directly that your DH was sending handwritten notes to this woman and she to him? Other work colleagues have also said the woman in question had some kind of breakdown before she left.
5 The work investigation - who actually told you about this? What evidence of it do you have, if any, that isn't hearsay? As PP have said, it would have been an investigation into sexual harassment rather than just 'did they, or did they not, have an affair'. If it was based on reviewing emails, an automated scan of emails will not have looked at handwritten images attached as PDFs to emails, just the emails themselves. But you wouldn't expect an investigation into harassment would have just been a review of emails.
So all in all a highly confusing set of evidence, and nothing actually provable beyond your DH being the victim of stalking. But you have some weird facts that either your DH has admitted or a workplace colleague has confirmed:
- he was handwriting non-work stuff in a notebook at work
- he was sending handwritten notes to this woman.
I think it is also telling that, in the counselling, all he did was protest his innocence. I suspect that he knew if he strayed from this one line he would eventually trip up on a lie. The purpose of the counselling was surely not to debate the facts but to talk about how the situation had made you feel, and by just saying 'but I'm innocent' he could shut that down and make it about him. (But mysteriously he also didn't want to talk about how he had been the victim of stalking?)
Add to all this your DH's obsessive behaviour around you (monitoring you?), the previous emotional affair, the isolating you from your friend to ensure that you would have no-one to validate your concerns, the fact he is now trying to shut down any discussion about it and telling you to move on. None of this points to a healthy relationship, whatever happened with this woman. But there is an awful lot of smoke here for there to be no fire.
I would focus on some more counselling for you alone, to reflect on your marriage and what you want to happen next. And probably also to come to terms with the fact you will never truly know what happened as too many people have told too many lies. But that you should find a version of it that makes the most sense to you, and act on that.