Muharram 4, 1330
1) The answer to your question is that Arabs apply the plural expression while addressing an individual due to the nice effect it produces [i.e. respect].
2) A testimony to this fact is what the Almighty says in Surat Al-i-’Imran:
Those to whom some people said: "A large army has been raised against you; so, fear them," yet it only increased their faith, and they said: "Allah suffices us, and He is the One upon Whom we depend most." (Qur'an, 3:173)
The person implied in these verses of Al-i-’Imran is none other than Na’im ibn Mas’ud al-Ashja’i, according to the consensus of scholars of exegesis, traditionists, and chroniclers. Yet Allah Almighty has applied to him, the singular person that he is, the plural form just to express respect for those who did not listen to his statements nor heeded his dissuading calls.
Abu Sufyan had given him ten camels in order to demoralize and frighten the Muslims regarding the strength of the polytheists, and he did just that. Among his statements then was: "People have gathered a mighty force to attack you; so, fear for your own lives."
Many Muslims disliked the idea of fighting that force just because of his statement, but the Messenger of Allah, peace be upon him and his progeny, came out accompanied by seventy cavaliers to meet them, and they all returned from the battle-field safely, whereupon this verse was revealed praising the seventy believers who came out with the Messenger of Allah, peace be upon him and his progeny, heedless to the dissuasion of those who wished to demoralize them.
In applying the word "people" for just one individual, a nice and divine point is made which is complimenting the seventy men who came out with the Prophet. This surely sounds more eloquent when used as such; it is better than saying: "Those to whom a man said that a large army had been raised..., etc.," as is obvious. There are nUmarous verses in the Holy Qur'an similar to this one, as well as in the Arabic language as a whole. The Almighty Allah says: "O you who believe! Remember Allah's blessing unto you when some folks intended to lay their (evil) hands upon you, and He protected you against their harm."
In fact, the person who intended to lay his evil hands upon them and hurt them was a man from the tribe of Muharib named Ghawrath - others say it was ‘Amr ibn Jahsh of Banu al Nadir - who unsheathed his sword and shook it intending to strike the Holy Prophet (pbuh), but Allah, the Almighty and the Glorified, foiled his attempt, according to the narration of the incident as recorded by traditionists, authors of chronicles, and scholars of exegesis, and as transmitted by Ibn Hisham in the campaign of That al Riqa' in Vol. 3 of his book titled Sirah. Allah has applied the collective plural "people" for this lone man just to express His blessings, the Dear One, the Omnipotent, upon the Muslim masses manifested in the safety of the Prophet, peace be upon him and his progeny.
In the Mubahala verse, He has applied both the singular and the plural forms to the "sons," "women," and "selves" to both the Hasanain, Fatima, and ‘Ali in particular, just to honour to their lofty status, may Allah be pleased with them. Examples for the application of the plural form for the individual wherever necessary are innUmarable and beyond recounting, and they all prove the license to use the plural form while talking about one individual whenever there is a nice eloquent effect thereto.
3) In his interpretation of this verse, in Mujma’ul Bayan fi Tafsir al-Qur'an, Imam al-Tibrisi comments on the usage of the plural form to refer to the Commander of the Faithful as a token of respect and veneration, stating that lexicographers describe the singular using the plural form to show respect and veneration. He says: "Sucn an application is too well known in their language to require proofs."
4) In his Kashshaf, al-Zamakhshari mentions another nice point when he says: "If you wonder how it can be accurate to use the plural with ‘Ali, may Allah be pleased with him, I will tell you that he is addressed in the plural form, although he is only one man, so that people may follow his example and earn rewards like his, and so that Allah may point out the fact that a believer's attitude should be like ‘Ali's, that is, being eager to do deeds of righteousness and goodwill by looking after the poor, so much so that even the performance of something which does not permit any delay, such as saying the prayers, should not make them postpone it till they are through."
5) I personally have a nice and more precise point. When the Almighty applied the plural rather than the singular form, as many do, then those who hated ‘Ali as well as all those who were envious of and in competition with Banu Hashim would not be able to tolerate hearing it in the singular form, for they would then be unable to hide the truth or water it down. Because of their desperation, they might even do something quite harmful to Islam. It is quite possible that it was for this reason that the verse was revealed in the plural form though applied to the singular: in order to avoid the harm resulting from disgracing those folks.
The verses after that particular one vary in form and status, gradually preparing them for wilayat, till Allah perfected His religion and completed His blessing, as was his usual habit, peace be upon him and his progeny, and that of the wise in attaining what otherwise is quite difficult to attain. Had the verse come in the singular form, those folks would have then put their fingers in their ears, covered themselves with their own clothes and become stubborn, arrogant, and naughty.
This is a sublime wisdom manifested in all the verses of the Holy Qur'an which were revealed to highlight the attributes of the Commander of the Faithful and those among his purified household, as is quite obvious. We have explained these statements and brought irrefutable proofs and obvious testimonies in our books Sabil al-Muminin and Tanzil al-Ayat, and praise be to Allah for His Guidance and Support, Wassalam.