What is the reason for the different vocalization of "الصابئين" (al-ṣābiʾīn) and "الصابئون" (al-ṣābiʾūn) in verse 17 of Surat Al-Hajj and verse 69 of Surat Al-Ma'idah, given the similar context of the two verses up to the word "النصارى" (al-naṣārā)?

The grammarians differed regarding the nominative case of the word "الصابئون" (the Sabians) in the verse from Surat Al-Ma'idah. The most probable opinion is that it is in the nominative case as an inchoative (subject), with an implied predicate. This is because when Arabs use a statement emphasized by "إن" (indeed) and intend to conjoin an anomalous conjunctive (ma'toof) to its noun (ism), they put it in the nominative case to indicate the conjunction of sentences, not individual words. Since the Sabians are furthest from guidance, their word was put in the nominative case as an alert to this fact. Al-Alusi and Ibn Ashour mentioned this estimation, and Ibn Ashour considers this estimation to be the clearest and most fitting for the style of the Qur'anic discourse. The fact that this word was revealed in this manner confirms that it is a valid and eloquent Arabic stylistic device, even if uncommon, indicating the conjunction of sentences. As for Surat Al-Hajj, the mention of the Sabians came in the accusative case and was fronted because the judgment upon them equates them with others in the فصل القضاء (decision of judgment) and judgment with truth, so there is no need for it to be in the nominative case, which is contrary to Surat Al-Ma'idah. One can refer to the commentators on Ibn Malik's Alfiyah in the chapter "إن وأخواتها" (Inna and its sisters) for further clarification.