Perhaps I am stupid but I fail to see in what way the Tuvix accident is related to organ donation. It's not like some evil doctor took Tuvok and Neelix and cooked a Frankenstein monster out of them. It was rather an accident that happened without the involvement of any agency, it was a symbiotic plant getting into the transporter.
I don't think you're stupid. I'm at a loss, myself, and I did my medical ethics thesis on the ethical issues in organ transplantation. Granted that was back in the mid-90s, but I've remained fairly up to date on the technology involved and the ethical issues. I have made reference to live donor transplants, but that's the only place I see any possible correlation, and the argument would benefit Tuvix.
It is, as you say, an episode that puts Janeway in a no-win situation. And the thing is, she knew it
. Look at her face at the end of the episode. She's not clapping Tuvok & Neelix on the back and smiling broadly at their return. She says a curt, "Glad to have you back" and walks into the hall where she pauses with a look that says the entire weight of the universe is on her shoulders.
She isn't happy with what she just had to do. She will, however, live with it.