If she was an evil monster who didn't care if her baby died, then she could have dumped it after the birth. Other mothers have done the same, just look at the newborn babies that have been found in bins or just dumped somewhere with cords still attached. She obviously cared enough to keep them with her for the best part of 2 months. My money is on coercive control from partner, PTSD and trauma from giving birth unassisted in a car ffs, and being on the run, maybe being told repeatedly by said controlling partner that the baby would be taken from her if they went to a hospital. Disassociation as a result of trauma meaning she buried the baby when it died and may not even have fully processed what has happened. Might be keeping quiet on advice of a solicitor. Trauma and PTSD really fucks the brain up and it's very difficult for ordinary people like those of us on the thread to fully understand why someone behaves in that way, but that doesn't negate what may be at play here.
Just my 2 cents. Fully aware they might just be negligent shitbags.