Is it know when or what events lead to Shepard earning the N7 designation? Are we to assume that it was awarded for which ever event the player chooses as part of their service history or did BioWare choose not to answer this question at all? For being the game's recognizable icon, there seems to be remarkably little information on the N-school or on missions which earn candidates the distinguished designation.
Mass-effect – When and how did Commander Shepard become N7
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In each game, there's an option to use a premade default Shepard. This Shepard is a Soldier, Earthborn Sole Survivor. If male, his name is 'John', if female 'Jane'.
Since this didn't change between games, I think we can assume it's as close to canon as anything (and certainly is in the same vein of the didn't-import-a-save options).
It appears you are a little bit mistaken - while there were Prothean Ruins (and technology) on Mars, the only Mass Relay in the System was orbiting Pluto; The so-called "Charon Relay" was discovered in 2149, only a year after the Prothean Ruins were discovered on Mars.
Given the level of advancement of humanity before the Prothean Ruins were uncovered, it was not unreasonable to assume that the Charon Relay was missed due to it being misclassified as an icy-moon. After the discovery on Mars, it became clear that both a) there were other sentient beings in the universe, and b) they were capable of constructing structures the size of very small moons, Humanity took a closer look at Pluto / Charon and said, "Huh. Fancy that.".
The Turians were not found on the other end of the Charon Relay, and the first contact war didn't happen until 8 years later in 2158, after Humanity was well on its way with colonization efforts (and had discovered and activated several more relays).
Best Answer
Most of the canonical information we have about the N-school and the N7 rank in particular comes from a BioWare ANN Blog Post. There's a lot of lore there, but the key points are:
By the time Shepard is XO of the Normandy, they are already a N7. Thus, the events that lead to that designation must have occurred before the first game.
Suspecting something chosen as part of the "pre service history" part of character creation seems logical, but looking at the codex entries for the various combinations seems to indicate that most of these events happened early in Shepard's career. Thus, I find it more likely that those events were the reason Shepard was recruited to the N-school, rather than the live combat exercise where (s)he received their N7 rating.
However, BioWare has never gone on record about this, so anything is technically possible.