Is a Sharia marriage contract to a Christian woman in a Muslim country valid if her guardian (her paternal uncle) later hosted the couple and implicitly agreed, and the contract was registered in the Real Estate Registry in the presence of her son, despite her distrust of the church and her later conversion to Islam?

For the validity of marriage, there are general conditions. Additionally, for a Christian wife, it is stipulated that she must be chaste. Her not following her church or not trusting priests does not affect the validity, as long as she believes in the Christian creed, because she falls under the general meaning of Allah's Almighty saying: (This day [all] good foods have been made lawful for you. And the food of those who were given the Scripture is lawful for you, and your food is lawful for them. And [lawful in marriage are] the chaste women from among the believers and the chaste women from among those who were given the Scripture before you, when you have given them their due compensation, desiring chastity, not unlawful sexual intercourse or taking [secret] lovers).

As for the guardian, if the Christian son is an adult, sane, and mature in matters of marriage, then he is the legal guardian according to the majority of jurists. If the marriage contract fulfills these conditions, then it is valid.