I am so sorry to hear that you are in this position. I hope that are looking after yourself well.
I will share my recent experiences in the hope that it will be helpful.
My dad died earlier this year and we had been NC for many years. His step-daughter contacted me just before he died to let me know he was dying.
With hindsight, I wish she hadn't. It caused a lot of soul searching and pain that was unnecessary. He had caused both me and my mum a lot of unnecessary pain and suffering for a long time in the past, and that was why I was NC.
People don't go NC without a reason generally. Prior to this news I did check the online archives of the paper local to him once a year, to loook at the death notices. That would have been the best way for me to find out. If there had still been any mutual connections, a gentle word from one of them would also have been fine. It would have been clean, simple, not at all messy and complicated.
I hope this will really be of help. My mum died several years ago, and it took a very, very long time for me to grieve. My grieving for my father has been much shorter and much easier. Partly because of being NC and partly because of the same reasons that meant I went NC.
I'll explain the latter more. Because of the type of person he was, it's actually been a relief for me that he is gone. That he won't pitch up, selfish and abusive, at random intervals in my life to wield destruction and chaos in my direction. As was his wont. I don't have that hanging over me anymore. There is also the fact he was a smaller part of my life, he was less loving, less supportive, he did me less good and more harm, so in a very real way he was less of a loss from my life than the type of father with whom it would have been possible to be on good terms, or have a loving relationship would have been.
What I have been able to do as part of my grieving, is, as you mention, do a bit of soul searching and thinking through some of the what might have beens. The what might have beens have plagued me on and off for most of my life. But the difference is that, recently, I have been able to let them go totally. It is proper closure.
I am sure you have been through a lot of agony and soul-searching about your relationship with your mother previously. Otherwise you wouldn't have been NC. I know I went through more than a lifetime's worth of that with my father. Now I realise that a lot of that agony and soul-searching was very similar to grieving, just done in advance of death.
So, however awful this feels now, there is a very, very real chance that you have done a, lot of the work of grieving, of coming to terms with loss already. And that this grief will be quicker and easier for you than it might have been otherwise. It is small consolation for all the past pain I know. But grief for someone whom you both love deeply and is an enmeshed part of your life is different I think than grief for someone who has caused you pain and suffered grief and been impossible to hold close, even if you do still love them.
I know that my grief and sense of loss for my father was absolutely dwarfed by what I felt and still sometimes feel for my mother. I know it was nothing compared to what I would have felt if he had been a loving and good father. You have felt a lot of the pain of grief and loss already, so whilst this is intense and awful for you just now, I really, really hope that your experience is similar to mine and it is naturally over quite quickly.