Wow, strongly disagree. It's taken them a good 18 months to turn into headshot experts. That is just radically different from your expectation that people should have turned into headhsot experts in the first 12-18 hours of the ZA.
Rick, a sheriff's deputy in Bung-Hole, Georgia who had been in a coma for some period of time just a couple of days earlier was popping off head-shots with his pistol like a pro in the second episode of the series as he's running from the tank to the alley Glenn meets him in.
Now, granted, it took the time between S2 and S3 for the surviving group members to become head-shot experts but, still, it's hardly something that I would think is easy to do outside of controlled conditions. It's not like they went to the shooting range every other day.
Military personnel, I would suspect, would be able to more accurate when it comes to aiming their weapons. Sure they're taught for "center mass" but they may be more able to strike the head than a civilian.
That said, I agree with your assessment on how the ZA probably first came down. Initially people probably weren't too concerned after a bite beyond cleaning and then bandaging it. Maybe after a bit some level quarantine. But, again, we've pretty much been shown that whatever happened happened pretty fast.