So I found a story about 'Joeseph Dahan,' who was found and released from inside an Egyptian diplomatic bag being sent from Italy to Egypt. Is a diplomatic bag with a person inside it inviolable?
No, a diplomatic bag with a person inside of it is not inviolable because it is not considered a diplomatic bag.
Article 27 of the 1961 Vienna Convention of Diplomatic Relations covers the protection of diplomatic correspondence, with sections 3 and 4 being the most relevant here:
3.The diplomatic bag shall not be opened or detained.
4.The packages constituting the diplomatic bag must bear visible external marks of their character and may contain only diplomatic documents or articles intended for official use.
So, diplomatic bags are completely protected from being opened or detained, but they may only contain documents. Since I'm sure nobody would consider a person a "diplomatic document" or "official article" then putting a person inside a diplomatic bag disqualifies it from protection.
However, if the person is covered in tattooed diplomatic messages they might be considered a diplomatic document...