It’s our firm belief that all Divine Messengers and Prophets are chosen by all and they were Prophet when they born. But many of them got the responsibility of propagation after having a specific age while others were introduced as a Prophet while they were in cradle.( e.g. Prophet Yahiya a.s. And Prophet Isa (a.s) and Quran clearly tells this)
The Holy Prophet who is the first ever creation of Allah above all creation( living and non living ) so when we talk about the sudden revelation of Quran means he had the Quran with him (when Allah created Adam (a.s), he taught him Names (Asma) which means the mysteries of the Universe).
Also whenever we talk about revelation of Divine command, it doesn’t mean something leave it’s place from higher position and come down. No it’s not like that as rain comes from up to down. Revelation means to make in thing comparable to others so they can have access to it. Incase of Holy Quran the reality of Quran is what no one can even imagine but when Allah sent it through archangel Jibrael to the Holy Prophet and he delivered those letters in form words so we Human can understand otherwise no one can say the Quran at Lauhe Mahfooz is in which form.
Also the revelation of Divine command is continue otherwise what does it mean by this Ayah which has use Present Tense for revelation of Wahi because Allah is eternal and His command is also eternal
تَنَزَّلُ الْمَلَائِكَةُ وَالرُّوحُ فِيهَا بِإِذْنِ رَبِّهِم مِّن كُلِّ أَمْرٍ
The angels and Jibreel descend in it by the permission of their Lord for every affair,(97:4)
But there must be a pure Heart which can have this ability to receive the divine revelation. Therefore our Imam Sadiq (a.s) says this Ayah is the best proof of Imam Zamana’s presence as Proof of Allah on the Earth because Earth can’t survive without a proof of Allah.
Secondly yes it’s true every Ayah has its Shane Nuzool ( reason of revelation) but because Quran is the book of Guidance therefore it must be according to the need and culture and faith and need of that period.
If I ask you what does it mean by “ Perish the hands of Abu Lahab and perish him”
While abu Lahab died 1400 years ago but here it shows a symbolic meaning that any one who has the nature or Abu Lahab this Ayah is for him right now today so you can say Quranic revelation is 24×7 continue and will be continue till Qiyamat.
When we read Quran as if Allah is talking with us that’s why there are so many Adaab ( manners) of the recitation of Quran.
أَوْ زِدْ عَلَيْهِ وَرَتِّلِ الْقُرْآنَ تَرْتِيلًا
Or add to it, and recite the Quran as it ought to be recited.(73:4)
وَإِذَا قُرِئَ الْقُرْآنُ فَاسْتَمِعُوا لَهُ وَأَنصِتُوا لَعَلَّكُمْ تُرْحَمُونَ
And when the Quran is recited, then listen to it and remain silent, that mercy may be shown to you.(7:204)
As you said if Quran has revealed before at once then at the time of each incident it means Allah know what is going to happen
Of course Allah know also these Zaman and Makan ( time and place) are for us otherwise if go deep in there is no Past and Future near Allah everything is present for Him.