What is the ruling on a woman observing a waiting period ('iddah') due to invalid marriage or unlawful intercourse?

Primary text

The opinion derived from the Madhhab suggests it is forbidden for the man who engaged in the invalid marriage or unlawful act, and others, to marry her while she is observing 'iddah'. However, the preferred view allows her to marry the man from whom she is observing the 'iddah', provided that the lineage of her child can be attributed to him, as the purpose of the 'iddah' is to preserve his semen and establish lineage. Therefore, what is forbidden (his semen) is not preserved for what is respectable (his lineage). Similarly, a woman divorced via Khul' is permitted to remarry the one who divorced her. Conversely, if the child's lineage cannot be established with the first man (such as in the case of fornication), he is prohibited from marrying her because it leads to confusion of lineage, placing him in the same ruling as any other man.