What is the grammatical analysis (اعراب) of the word "الصابئون" in Surat Al-Ma'idah, and why did it come with a "waw" (و) there, while it came with a "ya" (ي) in another verse despite the similar context?

The word "الصابئين" (the Sabians) appears in the accusative case in Surah Al-Baqarah and Surah Al-Hajj, and in the nominative case as "الصابئون" (the Sabians) in Surah Al-Ma'idah. Grammarians and exegetes have explained this difference in several ways, including fronting and delaying (taqdeem wa ta'kheer), or considering "الصابئون" as a subject with the predicate of "إنّ" (inna) being omitted, or as being an anacoluthon (atf) on the original nominative case of the subject of "إنّ". Ibn Ashour mentioned that the nominative case of "الصابئون" is suitable for the strangeness of their inclusion in the promise of forgiveness, due to the strangeness of their worship of stars. This draws attention to the greatness of Allah's pardon, which encompasses every believer in Allah and the Last Day who performs righteous deeds.