On the subject of instrumentalist egoism, survival vs. flourishing, and the pre-moral “choice to live”: someone just reminded me of Rand’s “Causality versus Duty”, which supports interpreting Rand as holding the less sophisticated views on these matters. Consider two quotations:
1. “Life or death is man’s only fundamental alternative. To live is his basic act of choice. If he chooses to live, a rational ethics will tell him what principles of action are required to implement his choice. If he does not choose to live, nature will take its course.” 
I think this supports the view of a pre-moral “choice to live”. It also suggests that living is understood here as the logical contradictory of dying (rather than as flourishing).
2. “Reality confronts man with a great many ‘musts,’ but all of them are conditional; the formula of realistic necessity is: ‘You must, if–’ and the ‘if’ stands for man’s choice: ‘–if you want to achieve a certain goal.’” 
This again lines up with the pre-moral-choice-to-live view. And I think it suggests that there are no ends to be found in nature, no goals that are metaphysically privileged. Rather, we simply choose what goals to pursue, and an action can be rationally criticized only when it fails to fit with the agent’s chosen goals. I think this stands in contrast to the Aristotelian view.
 Ayn Rand, “Causality Versus Duty,” in Philosophy: Who Needs It (New York: Signet, 1984), paperback ed., p. 99.
Ibid., p. 99.