Hi KT and welcome to the forum.
I'm sorry to hear that your Grandmother is so ill.
We are a group of patients and caregivers of patients with liver disease. My partner had a liver transplant over a year ago from Hep C, alcholic cirrhosis, and heptocellular cardinoma.
It is so odd that all the tests don't show the usual causes of liver disfunction or provide a clear profile of why your grandmother is comatose. Though it is interesting that she did better on the steroids...are they starting those again?
It is not unsual to need insulin while on steroids.
The fact that her other organs are working properly seems a really good sign.
Alcohol, viral disease, and autoimmune causes have been ruled out...so now what?
Is she being seen by a hepatologiest at a major medical center? If she is in a community hospital under the care of a general GI doc, her care needs to be upgraded. The major medical centers have the liver specialists..and she needs the best you can find.
Since I am not a doctor, nor do I have experience with the more exotic causes for liver failure, I urge you to research everything, including the material at the top of this forum, and to add a liver consultant to your treatement team!