Is a marriage concluded without a guardian (wali) or witnesses considered valid according to Islamic law, given that there are objections from the family related to the groom's beliefs?

The offer and acceptance you mentioned between yourselves is not considered a valid marriage, as the validity of a marriage requires the presence of a guardian (wali) and witnesses. Furthermore, it is not permissible for a Muslim woman to marry a man who holds apostate beliefs, such as reviling the Companions. Therefore, you must turn away from marrying him and obey your family.