For my money -- I really don't see how the merging of Tuvok and Neelix into Tuvix could possibly constitute anything other than their irrevocable deaths and Tuvix's birth, and I really don't see how the Tuvok and Neelix who emerged after Tuvix was split can constitute the same individuals who died. It's been ages since I've seen the episode, but it seems to me that the Tuvok and Neelix we followed thereafter are copies of the originals, not continuations of the originals.
By that logic, then, Tuvok and Neelix were already dead. Tuvix was not a Starfleet officer, and could not legally be ordered to sacrifice his life in service of the crew. As a result, it seems to me that Janeway and the officers who carried out Tuvix's "split" are legally guilty of murder.
Were I an officer in the 2377 Starfleet JAG office, I would definitely be looking to bring Janeway to court-martial for murder.
(Interesting side-question: Would Tuvix have been considered a Federation citizen?)