Didn't Tuvix have bodily autonomy too?
No since the amalgamated bodies belonged to Neelix and Tuvok to begin with. What I am saying is according to US law (and several other countries), all the rights belong to Tuvok and Neelix. The term is "continuous consent." Meaning not only to Tuvok and Neelix have to give permission, they have to continue to give permission. You cannot even use the body parts of someone declared legally dead without permission.
But we once again come to the transporter accident, they consented to using the transporter with the knowledge that something no matter how remote could go wrong with the transporter process. Something went wrong and as a consequence of their consent to use the transporter they have to abide by the consequences of their consententual decision to use the transporter.
Implied consent can only be taken if you can prove that both parties could reasonably expect to be put together in a transporter accident. Unless this outcome was so common that both parties were knowledgeable as to the risk, you cannot imply that their consent is based on an ambiguous idea that transporters are dangerous. You might reasonably consent to a danger of being killed, but I don't see how anyone could have a blanket consent that involves two different species and a blossom from a planet in the Delta Quadrant that hadn't been observed before.
In fact the only kind of consent documents I can see that Star Fleet might require is the consent for a Commanding Officer to decide your fate should you be incapacitated in the line of your Star Fleet Service, which is what we saw Janeway actually doing in the episode.
But if Tuvix isn't a separate entity and consciousness (with his own rights) and is simply a combination of Tuvok and Neelix then surely his demand to live can easily be construed as proof that Tuvok and Neelix are demonstrably giving their consent....if Tuvix is simply "them" combined then his desire to live is also their desire....and as long as he continuously claims to want to live, they are continuously giving consent
Actually if you watch the episode, in the beginning, Tuvix was also actively helping the Doctor to discover a way to separate him back into his two components. Could that not be construed to be a lack of consent on Tuvok and Neelix's part.
It is entirely possible that Tuvix was not emotionally stable in the first place, and that we actually saw the beginning of his breakdown before he was separated. But remember this is "continuing consent", and what we see at the end is that neither Neelix nor Tuvok asking to be put back together.