As someone who survived an abusive marriage, this Asakura character is a classical abuser scumbag.
1. why won't he kill the baby himself?
Because he has to be free of all "wrongdoings". He has manipulated Wakaba enough that he knows even if she got caught, she won't rat him out. Even if she did, it's an adult against a teen; a teacher against a student; a male against a female. He is in the clear through and through.
2. as someone as academically intelligent as Wakaba, why is she so "dumb" to obey everything he says?
This is how abuse and manipulation works. She was in a very vulnerable state, in her teens, neglected by her parents, thirsty for love and affection. She doesn't know that Asakura is only using her, because she never had a healthy relationship in her family. She doesn't want to get neglected again, so even though she knows what she's doing and asked to do is morally and legally wrong, she's going to do them anyway from fear of abandonment. Ironically, he will abandon her as soon as she becomes "unworthy".
The only way to clear this, is to get Momoko to just ignore the letter and run to the police.
But, she's also just a teenage girl who is getting manipulated by Asakura. So, what she can do is to confide in Kanzaki.
I really can't wait how Asakura will end.