All praise is due to Allah, and peace and blessings be upon the Messenger of Allah.
If a Muslim falls into apostasy and then wishes to return to Islam, they return by offering sincere repentance to Allah and pronouncing the two testimonies of faith (Shahada) while believing in their meaning and affirming them in the heart.
It is sufficient to say:
"I bear witness that there is no god worthy of worship except Allah, and I bear witness that Muhammad is the Messenger of Allah."
It is not a condition for the validity of one's return to Islam to add:
"He alone, with no partner."
However, if a person says it, it is good and virtuous, as this wording is authentically established in the Sunnah.
No.
The Shahada is not beneficial merely by pronouncing its words. It must be accompanied by faith in the heart, certainty, sincerity, acceptance of what it implies, and submission to Allah's law.
Therefore, the Shahada does not benefit someone who utters it with the tongue while denying it in the heart or rejecting the teachings of Islam.
No.
The essential testimony by which a person enters Islam is:
"I bear witness that there is no god worthy of worship except Allah, and I bear witness that Muhammad is the Messenger of Allah."
The additional phrase:
"He alone, with no partner"
is authentically reported in the Sunnah and is among the virtuous remembrances (adhkar), but it is not a condition for entering Islam.
This addition is authentically reported in certain remembrances, including the supplication after wudu (ablution).
As for the Tashahhud in prayer, a Muslim should adhere to the wording authentically reported from the Prophet ﷺ and should not consider additional phrases obligatory when they are not established in the Tashahhud.
Whoever wishes to return to Islam after apostasy should sincerely repent to Allah and pronounce the Shahada while believing in its meaning. It is sufficient to say:
"I bear witness that there is no god worthy of worship except Allah, and I bear witness that Muhammad is the Messenger of Allah."
It is not necessary to add:
"He alone, with no partner."
However, if one does add it, it is better and more complete.
And Allah knows best.