Even if Tuvix had claimed that he was best suited to represent Neelix and Tuvok's interests...and I thought that occurred during the episode...does anyone really think Janeway would have accepted that?
Also, people can't claim that Tuvix is merely a composite of Tuvok and Neelix -and- argue that he behaved in a manner that they would not. The two concepts are antithetical.
I would have found the episode stronger without Kes's emotional outburst, which to me played as more than a bit manipulative, especially given that a being's life was at stake. I'd like to think that if I'd been in Kes's shoes I would have been a bit more high-minded than, "I miss my boyfriend!!!"
I'll add that I think telling Tuvix "You're one of us...unless and until we -ever- figure out a way to restore Tuvok/Neelix, at which point you're raw material," is awful. Way to treat someone as a second-class citizen due to circumstances they're blameless for.