I agree, on-screen evidence is pretty conclusive that Spock was first officer in WNMHGB and Mitchell is at least senior helmsmen.
HOWEVER, I always wished Mitchell had been first officer and Spock had been second officer. Here's why: I think it makes perfect sense to put Spock in an inferior position in this pilot because it is a PILOT. Part of what happens in pilots is establishing the series. So, the pilot would have to answer: how did Spock become first officer? He replaced Mitchell.
Also, the scenes between Kirk and Spock play like Kirk is not used to following Spock's advice, something consistent with the "Mitchell XO" theory. BECAUSE the series had such a huge emphasis on Kirk and Spock's relationship, the pilot would have had to establish WHY. Why does Kirk trust Spock so much? Because of the events of this episode.
And, look at the way the dominos fall in the series proper, particularly with Sulu. Sulu is in physics in the pilot, but at the helm thereafter. Why? Because he replaced Mitchell!
It gave the series such depth to imagine Mitchell as this sort-of lynchpin to the crew. He was the first officer everyone liked. The crew had experienced a few adventures together, but not many yet. One thing was clear, though: they could always rely on Mitchell, the captain's best friend. Then, BAM! The events of the pilot turn that upside down and Kirk is left with a two-fold void: he needs a new first officer and he needs to be able to trust someone. Mitchell's downfall was clearly that his emotions overran his humanity, so who does Kirk trust now? Mitchell's opposite: Spock. Meanwhile, Spock finds himself in an unfamiliar position with a captain he isn't particularly close to. So, the series is about Spock and Kirk growing closer after the pilot. And, Sulu has to take Mitchell's place at the helm and, during the series, learns more about his own capabilities which eventually lead him to commanding the
Excelsior.
See? On-screen evidence aside, I think Mitchell as XO greatly improves the pilot and the series.
But, no, Spock was clearly XO.
