I give up on all the rest of the stuff because we are arguing in circles. Two things:
All I am maintaining about the killing of entities with souls is that the issue doesn't require much explanation. Buffy is able to kill entities with souls, just as you and I are, but she ought not, just as you and I ought not. At times, people on the show have to discuss this rule, just as human beings at times do, because there are occasions that seem to allow for the rule's transgression and Buffy may face more of those occasions than do most people. But nothing special is going on here and this discussion was only started by your original claim that Buffy "can't" kill ensouled entities, which was silly as worded, but was misworded.
You of course must believe in the Buffy world that unsouled beings nevertheless have essences, as Keith clearly does. If you weren't so interested in refuting for the sake of refuting you would see that claiming they have common personalities with their unensouled counterparts is to claim that they have essences. I was here in fact conceding to Keith part of his point, though only part. Sometimes you need to take yes for an answer.
What authors say about what they do and what they do are two different things. The show is better than Whedon's intentions, to the extent that you take a theological position about souls as seriously part of the show's intentions, which I do not, because the characters he writes pretty clearly display the kinds of differences you deny are there. I do not have scripts before me or a running memory of each episode, but I never read the Liam flashback episodes as you do, nor do I think you are reading them very well, but without their texts, and I don't have them, the argument can't go forward. I also agree with everybody else that arguments about Joss Whedon's beliefs are really not all that interesting, though I have certainly done more than my share of extending this thread.