DNA would only prove that Zawahiri was present. He could have only suffered a minor injury or could have been killed on the other extreme. DNA evidence is the same as fingerprint evidence, only DNA is more precise.
Not necessarily. For instance (and without being too graphic), let's say the DNA was extracted from a portion of his heart chamber or brain stem. I'm no physician or medical examiner, but I think I could make an educated guess as to his current status. :-)
Kind of depends on the piece you're testing though, doesn't it. I mean if you have, say, a plastic bag full of gray matter, you pretty much know.