What is the validity of the claim that there are scribal errors in the Qur’an, and that the word "وقضى ربك" (and your Lord decreed) is an error and the correct word is "ووصى ربك" (and your Lord enjoined), and that the word "قطنا" (our share/portion) has no meaning and the correct word is "فصلنا" (we separated/detailed)?

The summarized fatwa (definition):

The dispute regarding the recitation of "And your Lord has enjoined" (وَوَصَّى رَبُّكَ) instead of "And your Lord has decreed" (وَقَضَى رَبُّكَ) in Surat Al-Isra, and what was attributed to Ibn Abbas (may Allah be pleased with them both) concerning this statement, is from the weak and fabricated narrations. It cannot be used as a proof in matters of the Holy Quran. It has been widely reported from Ibn Abbas that he recited "And He decreed" (وَقَضَى), and the continuously transmitted (mutawatir) recitation is "And He decreed" (وَقَضَى), meaning He commanded. The word qada here cannot be understood in the sense of eternal pre-ordainment that prevents polytheism, but rather in its linguistic meaning of resolve and decisive determination. The Quran is preserved by Allah Almighty's protection, and it is impossible for any distortion or alteration to affect it.

As for the word "our portion" (قِطَّنَا) in Surat Sad, no one has challenged its Arabic nature or its Quranic authenticity. It is derived from "I cut the thing" (qatat-tu al-shay') meaning I cut it, and it signifies a share or a lot. It is also used to refer to a written document containing a prize.