Here are some thoughts,
- the fact that Leopold gave his address as the US consulate in Moroni does not necessarily mean that he was a diplomat. Most probably he gave that address for contact while he was residing there.
- the Comoros in 1971 were still part of France. That means that maybe you will have to search in French records in order to find more information about his death.
-According to his sister's obituary dated from 1965 Leopold's permanent residence was in Austria. It is more probable that his wife made the arrangements to have him buried there. Maybe search also Austrian records.
Good luck with your research.