@Becauseiam
Honestly reading your posts, to me you come across as a women who has lived in a domestic abuse environment for a long time and as such dont know how to separate yourself, your feelings, and your life from your husband.
One of the main behaviours evidenced in abuse is a continuing cycle of dependence, hot and cold.behaviour, uncertainty and excuses.
You need to support your child, if this isnt true (but unlikely as a adult making historic claims) you need to consider why they need to make such accusations. Are they worried about how you and any resident siblings are being treated by this man? Has the abuse been worse then you considered and as such they feel this man deserves his cupputance? Is there severe mental health issues at play?
It would be very unusual for a grown up to make baseless historic accusations as they are VERY difficult to prove and prosecute, and as such the adult gains nothing other then distress and further broken relationships, specially if the adult has almost no contact with the abuser anymore. It's much more likely for the abuse to be true and for them to be unable to live with it anymore and need to get it out, and tell people and try to get some support and justice.
If your husband was mainly cold to this child only I would support PP posts about it being potentially a cover for abuse going on as people would be more suspicious if he favoured the child, but also if he was abusing your child he may have justified his actions to himself by making excuses about the childs behaviour, as such causing himself to be cold to the Child, while giving himself the justification to abuse. It suggests that he saw the child as worthless, less then, and a belonging rather then a individual person in their own right.
From experience of a close friend, sexual abuse can happen while your in the house, a close friends daughter was sexually abused by her best friend while they where in the house, and she didnt find out until her daughter was in late childhood. It devastated her but proves it can happen, even more so when my friend never left her child alone with her best friend so it happened while she was in the house and she had no idea.