I think when it comes to Juhi Neil is always in his "guilt" mode. He feels like somehow he let her down or something. He also mourned her death, so seeing her alive again, he has become blinded to reality. He is thinking emotionally and not rationally. That is why after she was "saved" and she told him she was staying in RM for her "Jaan" all this time, and her Jaan is none other than her kid which is also Neil's, he believed her. She told him she got pregnant on the night of their engagement when they had too much champagne and since Neil didn't remember enough of that night, he could not refute her claim. She even said that she "Swears on him" that she is telling the truth.
The issue here is that the Neil that knew Juhi was not an ACP and sometimes when it comes to her he forgets to be an ACP and acts like 22-23-year-old Neil who's not the same as 28-year-old ACP Neil. (which is who Avni met). He acted like an ACP on a case with her from day 1, but he was getting emotionally involved with her, and when he realized this, he always got on the backfoot and kept suspecting her as a means of telling himself he is not "too attached" to his case. But when it comes to Juhi, he believes due to whatever she has been through she deserves every benefit of doubt.
In Avni's case, she is not being some typical FL going on "Detective Mode" saying that the ex is making a fool of you etc, but trusted Neil's word when he said he was the child's father and decided to help him "rescue" her. Today especially when I saw how Avni was dying over this child, I was SO mad, coz they all put their lives in danger so Juhi can get her kid back from HER father! This is the height! 😡