What are the opinions of scholars concerning the Disconnected Letters mentioned at the beginning of some sūrahs?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. They belong to the "mutashābih" (obscure) verses, the meaning of which only Allāh knows. [Al-Shaʿbī, Abū Ṣāliḥ, and Ibn Zayd] Abū Bakr al-Ṣiddīq said, "In every (revealed) book, Allāh has placed a secret, and the secret of Allāh in the Qurʾān is the openings of the chapters."
2. They are letters extracted from (divine) names, and when they are combined in certain ways, they form some names of Almighty Allah. ʿAlī ibn Abī Ṭālib said, "They are fragmented names; if people knew how to put them together, they would know that name by which, when Allāh is called upon, He answers the prayer." Ibn ʿAbbās was asked about (Alif-Lām-Rā), (Ḥā-Mīm), and (Nūn). He replied: "The name of al-Raḥmān (the Most Merciful) represented as initials." [Abū al-ʿĀliyah and al-Rabīʿ ibn Anas]
3. They are letters by which Allāh is taking an oath [Ibn ʿAbbās and ʿIkrimah].

Ibn Qutaybah said, "It is possible that Allāh is swearing by all of the individual letters, but only mentioned some of them, just as someone says, 'I learned "alif, bāʾ, tāʾ'' while meaning all the letters, and as some say 'I have recited al-Ḥamd,' meaning Sūrat al-Fātiḥah, referring to it by its first word. He swore by the letters of the alphabet because of their inherent worth, because they are the building blocks of the revealed books, and because He is mentioned and His oneness is declared through them."

Ibn al-Anbārī said: "The response to the oath is implied: 'By the letters of the alphabet! Allāh has shown you the path, and the evidences have been set out for you through the revealed Scripture.' It is left unstated because the hearers would understand this, and it can be inferred from 'That is the book in which there is no doubt.'"

1. The letters which are quoted point to all the other letters of the alphabet, as if to say that the letters are the building blocks of composed speech, and the Qurʾān is also composed only of these letters. [Al-Farrāʾ and Quṭrub]

One might ask: people certainly knew that it is composed of letters, so what is the point of drawing their attention to this? The response is that this is to highlight its miraculous nature, as if to say: "The Qurʾān is made up of the same letters you use for your own speech, so why is it that you are unable to come up with its like? When you fail to do so, you should realize that it is not the word of Muḥammad (ﷺ)."

1. These letters are names of the chapters (sūrahs) of the Qurʾān. [Zayd ibn Aslam, ʿAbd al-Raḥmān ibn Zayd, Abū Fākhitah the Mawlā of Umm Hāniʾ]
2. These letters are like the abbreviations used by the Arabs, like when one would say to another, "Hal tā (short for taʾtī)?", meaning "Won't you come?" and the other person responds "Yes". This is to use one letter to stand for the rest. [Al-Akhfash, al-Zajjāj, Ibn al-Anbārī]