I hope there's something in this comment that may serve you. Take what does and toss what doesn't. I wish only for your well-being, whatever that looks like to you, but I don't have any desire to influence or control you.
If they haven't listened before or changed their ways, then I think it's unlikely that it will get through to anyone afterward. So if you write to them, I think you should do it for you. But I think it's an unreasonable expectation that anyone will suddenly take responsibility for their actions, or start blaming someone else if they were previously made aware and didn't blame them. I'm not saying it can't or won't happen, but if that's your motivation, it's not something you have any control over, and they're not anyone you have any control over. What you do have control over is what it will do for you. I have a suspicion you were blamed for a lot of things that weren't your fault, and letters can be a way to have a final word and put responsibility back where it belongs.
If it helps to hear a different experience, I never stopped complaining about having been abused, but I was always shut down, temporarily shut out, etc. In my early 40s, I finally wrote and put responsibility on my parents for my mother's abuse (my dad enabled her) and asked for financial help because I needed treatment for injuries and for PTSD, and they said they would not help (and did not say but implied that they would not take responsibility). Several years later, I got back in contact. I said I love you and I forgive you. But, I did not drop my claim, and I did not deny the truth. So my mother wrote to me that "they didn't want to play the blame games anymore," said they loved me, and said goodbye from her, my dad, and even the pets! My point is, sometimes people will do everything to protect themselves from the truth, and do all kinds of crazy shit (things got crazier after they shunned me, my parents pretended to the world that we still had an emotionally close long-distance relationship). I know for my own parents, they can't face or accept that they experienced some extreme and scary abuse in their own childhoods, so they stand on false foundations; if they accept they abused me, then they have to accept the reasons they abused me, and their foundations crumble. They don't have the inner strength or the resources to manage truth, only false stories that make them feel better about the people they loved who hurt them and about themselves. I was supposed to own all that, become someone other than who I am in order to please them and fit their narrative, and believe in the narrative, too.
Guilt, on the other hand, which you brought up in your OP, I've come to understand through both learning and application is not an emotion, but a negating message with bad feelings attached. Guilt messages usually come up when we're wanting to do something for ourselves, and putting ourselves before others, especially when we truly need to. They're usually bullshit messages that feel very powerful and real. I've done a lot of work around all the guilt messages that came up for rationally choosing to not leave a suicide note for my parents, and every one of them was about protecting them and making things easier for them -- just as I was supposed to do my entire life as the scapegoat and scapemule who bore the blame and the burdens for my mother's abuse and all family dysfunction, even dysfunction in my parents' marriage from at least the time I was a toddler. (Actually, I was apparently quite Machiavellian and controlling before my first words.) Shame messages may also be attached to reinforce the guilt messages. Shame says, "You're not ________ enough," and if that doesn't work, it says, "Who do you think you are?" Or it might even say that first.
