Deuterium is supposedly poisonous. But it is almost exactly the same as ordinary hydrogen.
The Captain is not going to destroy a major base that he has been charged with bringing online based on the say so of a crew member who has been having hallucinations. That would be what is called a Career Ending Action.
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. However, it has been shown many times in Star Trek that one's title is not the only factor in outranking. Positional authority - that is, one's place in the command structure of a team, department, ship, or the fleet - is also heavily weighed when determining who outranks who. Even though the two may have equal ranks in terms of their titles, Una is the first officer of the Enterprise and has been for many years, while Pelia is only its chief engineer and Una's subordinate.
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.
Lt Kirk refers to Uhura as Ensign, whilst it was established earlier in Season 2 that she was not a Lieutenant.