Reddit mentions of Twice Dead: Organ Transplants and the Reinvention of Death (California Series in Public Anthropology, Vol. 1)

>So, I don't think this is plausible as an account of what death is, for two reasons. First, it would imply that a person is never dead so long as there is the potential for them to be brought back to life, and this seems highly counterintuitive (weirdly, it would also imply that Janeway never killed Tuvix, since they could have brought him back by replicating the initial experiment).

I don't think we're going to get to common ground on the definition of death here, as it's more contentious than it initially appears, and highly dependent on current technology, even in our own universe (Margaret Lock's Twice Dead would be a good start). Suffice to say, I find it more reasonable that Tuvix is not truly dead than that Tuvok and Neelix were, but this is all based on technology we don't actually understand or have a full ethical account of. Which leads to…

>Secondly, I think a strong case can be made to the effect that Janeway never actually brought Neelix or Tuvok back to life, but rather brought two new beings into existence. But I did claim earlier that what matters is the killing of one to save two, so the above isn't intended as a rebuttal, just a side note.

If this is true, then everyone who has ever transported is a new being each time. But that's an entirely different discussion.

Whether Neelix and Tuvok remained conscious/aware to a degree while their brains were combined into Tuvix's is irrelevant, since we accept that Tuvix is a discrete entity that makes his own decisions. His mental "software" is separate from Tuvok's and Neelix's, but he is running it on their "hardware" without their permission.

This is important to your final point, as to why Janeway can make the decision for Tuvix to sacrifice himself, but not for Tuvok and Neelix to sacrifice themselves. Ultimately, Tuvix does not have the right to make the decision at all. He doesn't have rights to his own body — he doesn't have bodily autonomy — because he doesn't have his own body. Janeway, acting as the state, has the duty to protect Tuvok and Neelix's bodily autonomy, but has no such duty for Tuvix.