I commend you for having the compassion to be willing to take on this special needs hatchling . . however like some of the others said, maybe it's just not meant to live. It may just have too many issues.
I know snakes and humans are way different from each other-- but I work in hospice and I teach families to follow the cues of their loved ones. As a person declines (and it can take months) they eat less and less. This is because the digestive system slows down. Some families try to force-feed their loved ones, but it often leads to GI distress and choking. Or they will give IV fluids and the body goes into fluid overload because the heart/kidneys can't handle it. Sometimes the body just knows what it's doing and all the love and care in the world won't fix things.
My other thought with this little snake is that maybe it also has cognitive issues that it may not be able to mentally process things and learn to eat on its own. You may have a long road ahead of you.
Since you have taken on this project and have taken it to the vet already and probably are attached to it, only you can decide when it has suffered enough and when to let it go. Or, hopefully it will indeed turn around, but it sounds like it may always need some intense help to keep going.
I hope you update us on it, no matter what the end result is.