Is it permissible to instruct someone in the Shahada and embrace Islam over the internet, and would the person thereby be considered a Muslim, given that some are of the view that written communication is not sufficient for this?

A person's embrace of Islam is not valid except by uttering the two testimonies of faith (Shahadatayn), due to many hadiths indicating the obligation of uttering them, such as the hadith of Mu'adh ibn Jabal when the Prophet, peace and blessings be upon him, sent him to Yemen, and he said: "Invite them to bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah." The People of the-Sunnah have agreed that the believer who will not abide eternally in Hellfire is one who has believed in the religion of Islam in his heart with firm conviction and has uttered the two testimonies of faith, unless he is unable to utter them due to an excuse, such as being mute.