I was mostly chaotic - good/neutral overall (bleeding heart but with tactical savagery) so I can't speak for how he reacts to an evil play-through, but my feeling also was that the less patronising you are to him, the more relaxed he gets. When he, for example, mentions his plans to ask Raphael about the scars and you list all the risks this might entail, he gets defensive, but if you just agree, he shows that he is aware of all the risks himself. Treating him as a person with a good head on his shoulders - unsurprisingly - seems to work best. Even after non-ascension, the options that related more to his situation than the morality of it have the nicest flow and sound like you are in it together.
And I just wanted to add, while I am not a fan of ascendant Astarion, I do enjoy reading the discussion. =)
