Is it permissible to utter the Two Testimonies (Shahadatayn) with the intention of entering Islam, out of fear of falling into apostasy, and is repentance from apostasy sufficient without uttering the Two Testimonies?

There is no justification for anxiety and confusion, for you are upon Islam, and you have not committed anything that would take you out of the faith. Cast away these whispers (waswas), for indulging in them leads to great evil. Uttering the two testimonies (Shahadatayn) is a good and required act for which a Muslim is rewarded, so recite them frequently as an act of worship to Allah, not because you think you have left Islam. All of this comes from the whispers of Satan to cast you into confusion and doubt. So whenever any of these thoughts occur to you, cast them away and say to Satan: "You lie, I am a Muslim monotheist." It is from Allah's mercy that He does not hold His servants accountable for the thoughts and obsessions that cross their minds. So repel these whispers, and you will be rewarded for repelling them, Allah willing.