Since the mother allowed the daughter to stay over on holidays AFTER the shower incident, then it seems highly unlikely that your husband had said or done anything inappropriate in the shower, as the mother is claiming. If the allegations were really true, why would she then allow her child to stay on holidays and continue seeing the father? That makes no sense. Given the timing of these allegations towards him, ie, when he tried to exercise parental rights, then it seems that the allegations were falsified in order for the mother to gain custody.

The mother sounds manipulative. It seems to me she only wants money from him and financial support, not shared custody, especially if things got ugly when taxes, custody, visitations, and family arose. You said she only approached him when she wanted financial support, after several years of carrying the full burden herself. So, now he wants parental rights, and she's fighting him on it by playing dirty.

I would trust your husband, and not this woman, the mother. Or, at least give him the benefit of the doubt unless proven guilty otherwise.