Why was the final alif of the word "ayya" omitted in the Almighty's saying: "And turn to Allah in repentance, all of you, O believers," and His saying: "And they said, 'O sorcerer!'" in the Uthmanic script? And does this have any connection to exegesis or variant readings?

The word "يا أيها" (O, you) is written with an alif in the Qur'an, except in three places: "يا أيها الساحر" (O, you sorcerer), "يا أيها المؤمنون" (O, you believers), and "أيه الثقلان" (O, you two weighty ones). This might be due to the sufficiency of the fatḥah (short 'a' vowel) to indicate the alif, or in consideration of the recitation of Ibn Amir, who pronounces the hā' with a ḍammah (u-vowel) in these instances, which is an Arabic linguistic feature narrated by al-Kisa'i and al-Farra'. It is also possible that these three instances collectively represent the different types of address (for the singular, dual, and plural).