Is the current marriage contract considered valid or void if it becomes apparent, after the engagement party, that the wife had previously been married and had a marriage contract without informing her guardian of that, and is she considered an heir and entitled to the Mahr after his death and her request for annulment from him?

The marriage contract itself is valid. If the husband stipulated the wife's virginity and then found her to be non-virgin, he has the option [to annul the marriage].

As for the guardian's request to annul the marriage due to the husband's kidney failure, he does not have that right.

If an agreement is reached for khul' (divorce at the wife's instance) in exchange for returning the dowry, then it is permissible, and they must return the dowry to the deceased's heirs. Scholars differed regarding the inheritance of a woman who seeks khul' from her husband during his death-sickness. The Malikis and those who agreed with them held that she inherits, while the majority (Hanafis, Shafi'is, and Hanbalis) held that she does not inherit. It is advisable to refer to the courts in such cases.

However, if the woman's guardian did not accept the khul' and no mutual agreement was reached until the husband died, then the woman is entitled to the dowry and owns it. She also has her right in the inheritance (one-fourth or one-eighth, depending on the existence of a child), and she must observe the 'iddah (definition) (waiting period). This is based on what Al-Tirmidhi and Abu Dawud narrated from Ibn Mas'ud (may Allah be pleased with him) concerning a woman whose husband died before consummation and before the dowry was specified: "She is entitled to the dowry of women of her status, she must observe the 'iddah, and she has the right to inheritance."