Re: Janeway in the cases of Quinn and Tuvix
Janeway does have the authority to order Tuvok to his death but doesn't have the authority to do so with Neelix. I'm not so sure whether she had the legal authority to order Tuvix to his death.
Even if she had the authority to (which I won't argue), that doesn't mean it was the moral thing to do. Also, under the Starfleet charter to seek out new life, even if she had the legal authority to execute Tuvix, that doesn't mean it was very principled to kill a new form of life or species.
Of course she has the authority to do whatever with Neelix, he's a member of her crew, and she is the Captain, without contact with her superiors, she is Judge, Jury and Executioner on her ship. Now, of course, if/when they make it back to Federation Space, she could be Court Martialed for any judgements/actions she took that Starfleet might disagree with, but, as long as she's top dog, it's her ship, and she can do as she pleases.
Now of course, this doesn't make immoral acts moral, it only states she is at this time, the ultimate authority on her ship.
However, I don't think her decision to save Neelix and Tuvok by sacrificing Tuvix was immoral.
Unfortunantly from a legal point of view she had no right to decide as she was emotional invested in the outcome. She should have recused herself until such time starfleet could be contacted so an impartial judge could decide.
After all the argument seems to be Tuvix had no rights, so what's the problem if he had to wait an indefinate time until someone impartial had ruled on the matter.
Tuvix had the same right to life as Tuvok and Neelix.
On the continent of wild endeavour in the mountains of solace and solitude there stood the citadel of the time lords, the oldest and most mighty race in the universe looking down on the galaxies below sworn never to interfere only to watch.