Is the marriage considered void if it was conducted secretly, in a car, without witnesses or a written document, under the pretext that the one who performed it was a Sheikh?

Marriage without a guardian or witnesses is invalid by consensus. It is not a condition for its validity that a shaykh or a specific person conducts it. Rather, it has conditions and pillars, such as the requirement of a guardian and witnesses. The woman should be invited to the methodology of Ahl al-Sunnah, and if she does not return to the truth, then her companionship should be abandoned, based on the Prophet's (peace and blessings be upon him) saying: "Do not accompany anyone but a believer, and let no one eat your food except one who is pious."