I'm also curious how you know so well where things were in their relationship. She's had plenty of time and space to brew some stories and convince even herself of a workable version of the truth. Or maybe she's telling the truth as it actually happened - highly doubtful, but who knows? The truth is a flexible thing to a cheater.
But hey, I'm just some guy on a forum, what do I know? In my case, I've been told there was no "sex" to this day - I got the Bill Clinton omission/lie, basically, until I asked very specific questions during a candid moment. With the things I know that happened and her mindset at the time, I'm sure posters would vote in the +90% range that it went all the way.
Does it matter? I'm not sure anymore... and she's not budging from her story. The truth I did manage to come by came only after I asked to separate. Her behavior in the beginning is as you describe your WW, but I was in no mood to tolerate it, and haven't been since. I wish I could also go back in time and un-tolerate her behavior in the years prior to the affair.
I would be careful not to completely disassociate what she did with reality - i.e. she went "psycho" - as it seems to me that this negates a very big and very real truth that both of you need to realize. She made not one choice, but a series of choices: She adopted a new mind-set which did not include you, and which actually caused and would have continued to cause harm to your life. Now maybe she wasn't acting like herself - my ww certainly wasn't - but I've never let her shy away from owning the many things she did wrong. She tried a lot in the beginning, but again, I don't tolerate it.
I completely understand the added sting of being ignored, having her slob around, expecting the world while doing nothing, while you are being faithful and good partner to no avail. Then she comes to life for some other POS guy who isn't 1/2 the man she has right in front of her. I get angry just thinking about it.
I also understand being close to personal grief/disaster and being abandoned in such a way by your partner, and I'm sorry to hear about your story. I was a better partner than she deserved, but then we were in in accident in which she was driving, and I am still recovering, and I was not/am not the person I was before. She stopped supporting my treatment as I had continuing issues, I stopped putting in the work at home and she hadn't been pulling her weight for years, and she had her affair about a year later. A few years before that, my brother died at 17 to a sudden brain disease which also affected my only other sibling. We named our son after my brother.
I understand being best friends, thinking she was incapable of cheating, and then watching her fall in "love" with some scumbag. Yes, it can happen to anyone. The punch that hurts the most is the one you don't see coming, and it seems that you and I both weren't looking in the direction of this blow. I also feel like I'm just going through the motions sometimes and I am empty, or at least not connected with my reality, as I take care of our son and work towards our future... my mind is 1/2 way out the door. Right there with you. Again, some of this is probably normal, but I think your situation could be improved.