What is the authenticity of the mentioned narration, and if it is authentic, how is the difference in Surat al-Nasr therein from the Masahif to be understood, and is that considered an alteration from the seven حروف (readings)?

The aforementioned hadith in the question, which links the revelation of Surat An-Nasr to the Prophet's (PBUH) saying to Ali and Fatima: "The help of Allah has come and the victory... Then glorify your Lord with praise and seek His forgiveness, for He is the Accepter of repentance," is a weak hadith. Even if we assume its authenticity, it is an interpretation and an act of fulfilling what Allah commanded. It is authentically reported in Al-Bukhari and Muslim from Aisha (may Allah be pleased with her) that the Prophet (PBUH) used to frequently say in his bowing and prostration: "Glorified are You, O Allah, our Lord, and with Your praise. O Allah, forgive me," and he would interpret the Qur'an, meaning he would act upon what the meaning of the noble verse "When the help of Allah comes and the victory" conveyed, by engaging in glorification and seeking forgiveness as he was commanded. The expression "The help of Allah has come and the victory" was also mentioned in another hadith from Ibn Abbas (may Allah be pleased with them both) at Al-Hudaybiyah, saying: "Allah is the Greatest, Allah is the Greatest, the help of Allah has come and the victory, and the people of Yemen have come."