Astarion is perfectly killable (tryed two weeks ago) ...
But they removed "Attack" button from his conversation ... so you have to attack him on sight. :-/ Or switch to other party member and attack him while he is talking to your "face person".

In the words of the senior NCO instructor at cadet battalion:
“If you ain’t cheating you ain’t trying. And if you got caught you didn’t try hard enough!”