Knowing more about the HOW of the zombie-pocalypse could really help us answer some of that. We've seen a few examples now of wounds that ought to be fatal healing up and people survivng that really shouldn't.
We could talk better to this if we knew it was a super-soldier serum gone bad, which is supposed to repair damage, and DOES so, but the other side of that coin is that upon death, it reanimates it as well. Attempt at cure for cancer, AIDS, whatever, take your pick.
Under those sorts of circumstances, some of the rest of this can start to make more sense, but we don't have a place to start from. Could even justify the whole world being infected by saying that the testing went great, cured stuff, etc. and just no one in the test group had died, so they didn't see the 'other' result until they started putting it out there after years of testing. As long as no one in the test group had died, or died without sustaining brain trauma at least, you'd never know it had the other result, just better healing, cure for X, no side effects.
Without a little insight into the PRE end of the world, hard to say much about what's going on now...