Why can't the rules of hadith terminology be applied to the science of Qira'at (recitations), such as deeming Ibn Kathir's recitation ("Gardens beneath which rivers flow") as anomalous because he alone recited it this way, and is it sound to claim that Allah uttered every letter of the recitations despite their variations?

The recitation of "تجْري من تحتها الأنهار" (rivers flowing beneath them) in the verse from Surat At-Tawbah is not exclusive to Ibn Kathir; rather, it is a common recitation in Mecca and is written in their Uthmani Mushaf. The acceptance of a recitation requires it to be mutawatir (transmitted by multiple chains of narration), in conformity with the Arabic language, and consistent with one of the Uthmani Mushafs, even if by estimation. Tawatur means narration by a group that establishes certain knowledge. The recitation of the ten Imams (Abu Ja'far, Nafi', Ibn Kathir, Abu Amr, Ya'qub, Ibn 'Amir, 'Asim, Hamzah, Al-Kisa'i, and Khalaf) combines these three pillars and is considered definitively established.