Interesting rationalizations, and I agree, having seen only six episodes and being largely sheltered from any larger view of the world we can't dogmatically argue one way or another.
Two nitpicks however. The US Strategic Petroleum Reserve. the largest in the world, has a 160 day supply of oil on hand. I think it was mentioned at some point that they were at day 60 something since the zombie thing started? There should be plenty of petrol around, assuming distribution(a big assumption) continued.
As to headshots being the only way to stop a walker. Unless the CDC doctor was keeping physical changes to walkers a secret, the only change is in the brain stem. That means over-pressure(which is the the main way bombs kill people) will kill walkers as sure as a headshot. Whether the brain is turned to mush by a bullet or over-pressure, the end result is the same. Also, limb wounds and such might not kill a walker instantly, but they still have blood, they still have internal organs that require certain processes to work. Just because walkers are governed by pure instinct doesn't make their biology any different from ours. They'll bleed out just like any other human.