Deuterium is supposedly poisonous. But it is almost exactly the same as ordinary hydrogen.
Una says she outranks Pelia, but she is a Lieutenant Commander and Pelia has been called Commander multiple times so they are at least the same rank.
Seems to be a huge error in judgment on Kirk's part to give a phaser to Uhura knowing she is hallucinating, especially after she had already punched him due to a hallucination.
Deuterium is the other stable isotope of hydrogen, deuterium is formed when a stray neutron bonds to the proton in the nucleus of a hydrogen atom (protium), hence its nickname of heavy hydrogen. Like protium, deuterium is highly flammable, so a half dozen photon torpedoes detonating inside a deuterium rich nebula, and destroying a refinery filled with enriched deuterium, would ignite the entire nebula. Which would destroy the Enterprise, the Farragut and everything else in the nebula.