What is the reason for the difference in reading a single word in Surah Az-Zukhruf, verse 19, between the recitation of Warsh and other recitations, despite the Qur’an being preserved from distortion?

The Quran is preserved from alteration because Allah the Almighty says: "Indeed, it is We who sent down the Dhikr (the Quran), and indeed, We will be its guardian" (Al-Hijr: 9). The exegetes have unanimously agreed that "Dhikr" refers to the Quran.

The Quran was transmitted by tawātur (mass transmission) from the Prophet, peace and blessings be upon him, who received it from Jibreel, peace be upon him, from his Lord, عز وجل. And it was transmitted to us by people of tawātur, who cannot possibly conspire to lie.

As for the multiplicity of Qirāʼāt (readings), this is not an alteration; rather, these are readings transmitted by tawātur from the Prophet, peace and blessings be upon him. All of them are part of the Quran, there is no contradiction between them, and they convey valid meanings. An example of this is the two readings "عِندَ الرَّحْمَـٰنِ" (with the Most Merciful) and "عِبَادُ الرَّحْمَـٰنِ" (servants of the Most Merciful) in Surat Az-Zukhruf. Both are correct and mean that the angels are servants of Allah and are with Allah.