Create a timeline with the accident, first examination, later findings, incapacity, correspondence and knowledge of possible wrongdoers. For each injury, note when it was first identified and how treatment developed.
Then separate four questions: Who caused the accident? Which injury resulted? Which consequence is attributed to the missing belt? When were loss and wrongdoer known? This prevents contributory fault and limitation from becoming one undifferentiated objection.
A legal assessment requires accident records, medical development, information about belt use and the complete correspondence. The more precise the chronology and allocation, the more clearly the claim can be assessed.