Short answers, pulled from the story.
Classical Arabic developed as a relatively uniform intertribal poetic koiné by the late 6th century AD. This literary style emerged as a synthetic language distinct from spoken vernaculars and included conservative features like case endings known as i'rab alongside innovative elements.
Classical Arabic preserves the complete Proto-Semitic three grammatical cases and declension system known as i'rab. It retained 28 out of 29 evident consonantal phonemes making it a conservative language among Semitic tongues while maintaining historical continuity in morphology.
Arabic grammarians had standardized the language by the 2nd century AH which equals the 9th century AD. Sibawayh developed prescriptive grammars during the Abbasid era that facilitated understanding of linguistic aspects and established the standard literary register for Classical Arabic.
Old Hijazi features such as loss of final short vowels influenced the consonantal text of the Quran during the 7th century AD. These phonological changes contributed to the development of later normalized orthography that emerged in the 8th century as a standard literary register.
Joshua Blau argued that differences between classical and spoken language were not too far-reaching despite common assumptions. The region developed into widespread state of diglossia where classical language coexisted with spoken vernaculars across Islamic world and pronunciation likely influenced by vernaculars to different degrees similar to Modern Standard Arabic today.