What a terrible tradgedy for everyone involved.
This is a loss for you, and you need to heal away from your grand daughter for a little while, take that time. It may be hard for you to let it go, which is sad for her, because you sound like her best hope for any stablity. If you can let go and not resent the death of your beloved dog, then she really needs you. If that happens, and I hope for her that it does, then you should start thinking of her not as a and almost 5 year old, but as a child who has a developmental disability, which means that she is not almost five in her ablity to handle most situations, and she is not ready to be left awake without direct supervision for her own good. She could have drown herself, but you were lucky, as sad as the situation was.
If you are able to let this go and be a care giver for your grand child again, some big changes need to go on, for her sake. Your daughter and her husband need training in the worst possible way, but so do you. Contact a social worker at her school and get some training for how to care for a child with ASD and ask for help to assist your daughter and her husband with the same. She needs specialized care; all the screaming in the world at a deaf child will not help, any more than strategies that work on nuerologicaly typical kids will work on a nuerolgicaly atypical child without some theraputic intervention. That you use euphamism (not listening) to describe her behavioral problems is a dead give away to a great deal of what is going wrong here, and your irritation at how poorly she responds to what you know works for neurotypicals is obvious. Know too, that your daughter and her husband may not be doing what needs to be done here, but even if they were doing everything you were, the results would not be much better.
Bame is not going to accomplish anything for your grand daughter. Even if the parents are doing less than you are, that will not improve things, and as a parent who has been judged to have never done anything to control my autistic kid by her own parents, I can tell you that all that stuff that I did made so little difference that it is very easy to conclude that I have never lifted so much as a finger while I was exhausted from all that I did every day. I can see how someone could give up on trying, because it is that hard, which I think you know, since there is very little that is working for you either, so let that go.
What you are describing is very typical for parents of an autistic youth; very few marraiges survive it and considering the stress of both an autistic child, and a rocky marriage, they are even less equiped to help her as time goes by. Either issue would be more stress than any one person can handle. They need help.
Start looking at this from the perspective that each of you is doing the best you can, and just maybe your best is better at this point, but that does not need to remain the case. Once you have the training you need, see what you can do to convince your daughter and her husband to apply structure to her world too. They may just be overwhelmed and need help to do so. If the structure in both houses are the same, she will have a chance to progress.
If she is not in private therapies, and a lot of them, she is not being treated appropriately. School is great. Schools are required to identify children with disablities and serve them such that they are "functional" in the school setting. My guess is that the school has done that, but they are not responsible for all her treatment, and they are not going to maximize her functionality at home. She should be in private speech, occupational, play, cognative behavioral, and medical therapies. As you know already, sleep is often disurbed in children with ASD, and medical intervention to ease a sleep wake disorder is essential for her to be learning and making progress.
Urge your daughter and her husband to access these therapies right away. If you can attend cognitive behavioral and play therapy with her, you could learn some new tecniques for potty training and how to use concrete langague that she will understand better. If this child has not been to a developmental pediatrician, see that she goes ASAP.
My heart goes out to your whole family, especially your grand daughter. As devistating as this is for you, it is really one of the saddest stories I have ever heard about the effects of under treated autism. She will have to live with this her whole life. I hope that you all can make this the wake up call it could be, and find some help for you as a care giver, your daughter and her husband to cope with thier marriage, parent training for your daughter and her husband, and more therapies for your grand daughter so that she can make some much needed progress.