Is it permissible to conclude a marriage contract without public declaration, with only the woman's son and his wife being aware of and witnessing the contract, due to the couple's unwillingness to publicly declare the marriage because of social norms that view negatively the marriage of an older woman to a young man, and is this number of witnesses sufficient in the absence of an officiant?

It is not a condition for marriage to be conducted by an authorized officiant (ma'dhoun), but if only the wife, her son, and his wife attend the wedding, it is invalid due to the absence of a guardian (wali) and a flaw in the witnesses. The son cannot be a valid witness if he is also the guardian, as he would be accused (of bias), and according to the preponderant opinion, the testimony of women is not valid (for marriage contracts). Furthermore, disapproving of a woman marrying someone younger than her is incorrect.