I consider him as a damned soul entrapped in a dead body.
About cellular reproduction and so on: in this world metamorphing is a feature that happens not through biological means but by the use of magic, pretty sure that can explain a big part of the tadpole's host transformation in a mindflier (let's not forget that those worms can infect all races and transform them, something that from a biological point of view is quite complex to achieve due to the fact that even if they are humanoid they still have different set of chromosomes. Truth be told I justify the existence of the weave as the reason there can be half races)
About eyes and skin color: Larian decided to give full spectrum of skin tones and colors, same for eye colors and hair color. That is why I assume people in this particular version of Faerun is used to unusual somatic aspects: my full Drow has GREEN skin and hair, and DEMONIC ORANGE still he is recognized as an appartenent of his race and even other drows don't give a dime, in the post with character pictures we can see a variety of non standard pigmentations, hair colors, eye colors, but as far as I've read no player has writen about their character being somehow harassed or pointed out because of their aspect.
About Astarion smiling, I don't know about others people habits but when I talk with someone I don't look to their teeths.
