What strikes me from your posts OP is the hostility towards your mother, and how you're certain that your father was innocent and blameless.
If I've understood correctly, the outpouring of emotion and stories that you've heard are from your father's side of the family. They're hardly unbiased, and they're very, very unlikely to want to paint your mother in a good light.
You haven't given any details of why your childhood with your mother was so bad, and it's not our place to ask about this. But you said until recently you didn't realise it wasn't normal, so that suggests that she was unavailable, uninterested, etc. You've also referenced the fact that your grandmother, your mother's mum, was a pretty awful person and that was the reason that she married.
As a child, it's really hard to get past abandonment, even when it's temporary. And you clearly still have raw scars from that now that you're trying to heal.
I think multiple things can be true. You can be understandably struggling with the period when your mum walked out and you only had visits with her, rather than living with her as you'd been used to. You can also struggle with the fact that she was detached and emotionally distant from you. But, I wonder what story your mum would tell, if she was able to talk without fear of judgement or shame? It sounds as if her life has been pretty awful, and the fact that she fought to get you back suggests that she wouldn't have willingly chosen to walk out unless she was at breaking point for some reason. And possibly her own scars are why she was distant emotionally.
I'm not suggesting you get back in contact with her. If NC feels right for you, then that's great. But sometimes you really start to heal when you let go of the bitterness and resentment, and try to understand what might have been going on for the other person. From a bystander's view, it sounds as if she's spent a life struggling, isolated and unhappy while your dad was surrounded by an extended and loving family and was "bonded" - to use your word - with his children. It sounds as if she might have felt like a perpetual outsider, with no way of forgiving herself or forging genuinely loving relationships.
None of that invalidates how you feel now, but it might help you to let go and move on if you can understand how maybe it was less of a conscious and deliberate choice for her, and just a woman struggling with life.