Is a woman sinful if she does not wear the niqab, and is it better for her to wear it sometimes or not to wear it at all, and is a woman who wears the niqab sinful if she does not cover her forehead?

A woman must cover her face from men who are not her mahram. It is not permissible for her to be lenient in this, for uncovering her face in front of them is an act of disobedience to Allah the Almighty. Repeated disobedience increases the sin, and it is obligatory to abandon it completely.