Is it valid to conclude a marriage contract online via audio and video, with the presence of two witnesses and both parties or an agent for one of them (other than the guardian), along with the completion of the offer and acceptance formula?

The most authoritative opinion among jurists is that a guardian (wali) is a condition for the validity of marriage. It is not permissible for a woman to marry herself off or to appoint someone to marry her off. Nor is it permissible for a Muslim to choose between schools of thought (madhahib) seeking a concession. A marriage conducted through modern communication methods is not valid. The appropriate alternative is for the woman's guardian to appoint a just Muslim to act on his behalf in the marriage contract. If a lawful marriage is not possible, the relationship must be severed because she is a non-mahram. Marriage via the internet may end in failure due to not knowing the true nature of the other party.