^ (1) Headshots. Gotta luv 'em. But as we saw in the final episode, only the brain STEM appears to be reactivated by the zombie-causing virus. That leads me to further believe that a headshot might not actually destroy one, depending upon shot placement. We see our heroes shooting their way through groups of Zs and dropping them with headshots, and I can imagine the hydrostatic shock jarring the brain stem enough to drop the Z in such a manner, but we don't linger on the scene long enough to see if the Zs who didn't suffer a traumatic brain STEM injury get back to their feet or not. That is food for thought.
And all of the sights you mentioned are most excellent. But you're firing on an erratically moving object that may or may not be moving predictably...the Z in question might have a leg or foot injury that makes it limp from side to side, or may be missing a limb that throws it off balance for a hard kill. Uninjured insurgents are a little more predictable to shoot, and a center-mass shot is sufficient for that purpose. Zs don't drop with center-mass shots. You can focus on the "healthier" Zs who don't limp or who move slower, but that doesn't deter the gimps who are homing in on you while you make the easier shots.
(3) Number of bullets...I agree that it would be a highly-sought-after skill set the day after a zombie contagion outbreak, but if you know how to handload your own ammo, odds are you're already well set-up to handle this kind of a situation, or can be in short order, and thusly your skills aren't going to be made available to the general public. And as it's been pointed out elsewhere, the gun stores and pawn shops are likely to be (1) completely empty or (2) barricaded and loaded for bear and not letting anyone else in. Well, perhaps Andrea or Lori could, ahem, "barter" for something to gain admittance, but the rest of the group would be SOL...
(4) Logistics...good idea! The HEMTT is awesome. But in addition to all of the above problems, I think you'd see guys taking HEMTTs for themselves, to go looking for their families. I think you'd see desertions; certainly you'd se significant breaks in the logistics chain at various points as areas became overrun and began looking out for themselves. A Cav unit defending Atlanta against several million Zs might be written off.
And the most important distinction of all...our disorganized, infighting, leaderless, ill-equipped heroes are all on their own, no help, no guidance, no Emergency Broadcast System instructions, little sleep, constantly on guard against walkers, just barely hanging on to their sanity and their lives. There's no government agency coming to the rescue, no one saying "Go (here) for help". We are seeing everything as it is happening to them, with no significant lapse of time in between episodes; everything that is happening, from the time Rick finds the group to the events of the CDC, appears to be in the span of 48-72 hours. It's a lot for a group of civs to overcome.
For the record, I wholeheartedly agree with you about wondering of the fate of the military. We see a lot of stopped tanks and vehicles, but not a whole helluva lot of dead soldiers. It does stand to reason, logically, that the military must have written Atlanta off and retreated to parts unknown. Benning? Perhaps. Maybe we will find out. A throwaway line explaining their abscence would go a long way towards helping the "suspension of disbelief" that is so important in the weaving of a tale like this.
But the only guy who seemed to know anything, anything at all, was Dr. Jenner, and he seemed very disinclined to talk about conditions in the outside world in any kind of great detail, whether because he did not know or did not care. For what it's worth, I can surmise that he did not know the details because it wasn't pertinent to his work in the CDC; maybe someone alive on the outside could make use of his work and maybe not, but he was too busy studying the virus and effects to be watching CNN. As a scientist, I doubt communications protocols or fixing a broken satellite dish or whatever was preventing him from reaching someone was within his skill set. And he had bigger fish to fry.
Hope this helps.