“where (will) the nitrogen component will come from, if the atmosphere is not to be essentially O2 plus CO2.”
Even if the atmosphere is not breathable by non-genetically modified humans, just having the pressure of an atmosphere could allow people to get around with lighter equipment, like a scuba mask and backpack re-breather, rather than heavier/bulkier dry suit style full pressure suits, like astronauts. It would also ease the design considerations and emergency equipment required to protect against leaks or breaches of buildings on Mars.
As it is, the pressure differential would kill people before they would suffocate. It is almost the vacuum of space - less than 1% of Earth pressure.
More significantly, it might allow open agriculture or forestry, as a method of generating a variety of resources. Crops might need nitrogen, but they might also play a role in sequestering nitrogen - possibly using specially created plants, algae or fungi.
Also, even if the atmosphere is not breathable, it would have a strong effect in moderating the wide daily temperature swings that must be dealt with - currently a 170 degree F daily range on average. Like the pressure, such temperature specs require a whole host of expensive/difficult/dangerous structural design requirements and specialized equipment.
The electromagnetic shield in orbit concept described in other posts, could also reduce one of the other challenging global environmental factors on Mars - radiation.
If temperature, pressure, radiation and moisture were made Earthlike, those would be big steps in easing life support generally.
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Pipe dreams are more realistic than your suggestions.