The key word there is sentient, Tuvix as a sentient being was entitiled to all the rights granted sentient beings by the Federation. If he chooses not to give up his life, well that's his right. People die all the time in accidents.
As for Janeway being promoted to an Admiral, it's quite clear she was a menace to the space lanes, so she was promoted into a position in which she could do least harm. Conversely Kirk was demoted into a position in which he good do most good.