Why do these details matter so much ?

Brain atrophy is impossible to image in the early stages. It is usually noted by comparison images taken a decade or more apart.

A DTI MRI can show evidence of trauma. Diffusion Tensor Imaging looks at the water flow in the brain and can indicate where that flow is abnormal. This usually indicate a response to trauma/injury.

As I said, Brain stem concussions are not mentioned much. They would likely heal like any other concussion.

Every brain injury is different. Extent of damage and length and completeness of recovery is different with each individual.