If Tuvix really had the qualities of Tuvok and Neelix, he would have freely volunteered his life to save both Tuvok and Neelix.
That he was unwilling to do so demonstrated, concretely, that he was as a mythological changeling, as a troll exchanged for a human infant, but, in this case, two.
For all intents and purposes, he was a manifestation of the abduction of two crew members, lacking the character of either.
Additionally, as the degradation that he was, in the long run he was even potentially a security threat to the ship.
So if he had volunteered to get whacked, he would have passed some sort of cunning ruse and been allowed to live?
Neelix is a deserter who ran a way from a war and people depending on him.
If Neelix really cared about Voyager and Voyager's crew he never would have found a girlfriend, abandoned Voyager and settled down.
Tuvok has a fundamental belief in the right to chose suicide if the situation calls for it.
In The Void Tuvk aruged that voyager should kill rape an pillage from weaker ships in the Void for Voyager to survive.
What you call Tuvix's Trollish behaviour, I say is unique characteristics that proove that Tuvix is a distinct being greater than the sum of his parts and a real living feeling being with a soul having the same complete set of rights as everyone else on Voyager.
If Neelix really was so damn necessary, then Kathryn should have ordered a less important lower decksman, certianly a Maquis who is barely only qualified to push a mop, to hand over a lung back in seaosn one.