Cohort commander?

[popularis]

Perhaps having a vir militaris family helped boost a young man into the officer's club?

The straightforward answer to this is a definite yes! There are a bunch of young men in equestrian positions identified by Kleijwegt (Ancient Youth, 1991), and a 'military' family is, IIRC, the main common factor. They'd have social rank, great connections, and a decent mentor - not a bad position to start from.

... half or more than half the strength of a consular army was composed of allied cohorts with each of them having a single commander, ...

I'm not sure that that's true under the Republic.

It's not. So far as I can tell, Praefecti socium acted as a college, same as military tribunes: they could be assigned command of a cohort/unit in a given situation, but that was a matter for the commander at the time. Add that to the fact that the alae sociorum were composed of units of differing sizes from different cities/peoples, and you can see why the command structure had to be flexible.

There's another logic to all of this. Allied units in the Republic were, in general, raised, managed and paid for by local elites. Those elites derived status and power from that military command. Gallic or Spanish tribal chiefs needed to command their people in battle as much as Roman politicians needed to show military prowess. It wasn't in Rome's interest to undermine their friends in their home communities! Nor would it make sense to assign a Roman commander to a group of men who might well speak a very different language and have their own way of fighting.

The last point is less important for the Italian socii and latini - who seem to have integrated into the legions pretty well post Social War and may well have been armed and arrayed similarly to the Romans by the late 1st century BC - but it would certainly have applied to foreign auxiliary units.