I've heard that one of the limiting factors on human strength is pain, and since walkers (presumably) feel no pain than it is feasible that they could be stronger than ordinary humans, no? And remember that in Ep2 Mark said that in the opening days it spread quickly, as in all over the country. I'm fairly certain that if that were the case the military wouldn't stand much of a chance. All bases would be under attack at once, and not a single soldier would know how to bring down a walker. Sure, they'll probably find out that head shots do the trick but they have no idea that the bites kill, and after death, no matter how you die, you'll reanimate if your brain is intact. Bases that held out against a seige could fall from the inside when dead comrades rise and nobody knows what's going on.
The virus spread quickly, too quickly for the military to effectively combat or contain is what it seems like happened. I kind of agree with you about the whole "everyone dying" aspect, but in reality we'll all die from old age anyway, even though "old age" is likely to be somewhere around 50-60 with walkers everyone, if that. As for borders and quarantine, who are we to say that the entirety of the world has fallen? We actually have no idea about the situation outside of the United States, or even within since we'll never be going further west than Georgia. While the situation seems bleak, and saying that rest of the world is in the same state is likely an accurate conclusion, we'll never truly know. For all we do truly know Canada and Mexico could be high-fiving each other for successfully containing the outbreak inside America, and saving the rest of the world.
And, I don't get what you mean by zombie inflation. Could you please explain that?