When I saw the photo of all that black tissue, if that had been my chicken, I would probably have euthanized right away. I love my chickens to a sickening degree, and I will try everything I can to make a sick one well. But when there is a sign such as that necrotic tissue, it tells me infection is likely so advanced, nothing will probably help.
You did everything humanly possible for your hen. Guilt should not complicate your grieving for her.