PART 1
This sort of things (changing the point of view, altering between people being described, and etc.) is common in Quran, and not common in regular conversations of any language, including Arabic, as far as I have seen. However, Quran is in Arabic but is not a regular script. I remember a scholar saying what is hidden in Quran is far more than what is obvious and visible therein. Those hidden parts of Quran are then discoverable from the visible parts, based on evidences. This is not a personal idea, I have already seen several Ahadeeth explaining it, and it is also talked about in Quran itself, like when Allah says إِنَّهُ لَقُرْآنٌ كَرِيمٌ . فِي كِتَابٍ مَكْنُونٍ . لَا يَمَسُّهُ إِلَّا الْمُطَهَّرُونَ, which translates into That this is indeed a qur'an Most Honourable . In a Register well-protected . Which none shall touch but those who are clean, so that not all people can understand every secret implicit in Quran and they should ask those who are clean to discover and explain the secrets to them: فَاسْأَلُوا أَهْلَ الذِّكْرِ إِنْ كُنْتُمْ لَا تَعْلَمُونَ which is translated into So ask the people of the message if you do not know. The people of the message are the holy prophet PBUHH, himself, as well as his household PBUT. The holy prophet PBUHH is among the people of the message, because first of all the message is Quran ( إِنَّا نَحْنُ نَزَّلْنَا الذِّكْرَ وَ إِنَّا لَهُ لَحَافِظُونَ) and no one is closer to Quran that the holy prophet, and also because not only Allah revealed His words to the holy prophet PBUHH and selected him to be the messenger, but because also introduced him as the interpreter of His sayings:
وَ أَنْزَلْنَا إِلَيْكَ الذِّكْرَ لِتُبَيِّنَ لِلنَّاسِ مَا نُزِّلَ
إِلَيْهِمْ وَ لَعَلَّهُمْ يَتَفَكَّرُونَ ...
... And We revealed to you the message that you may make clear to the
people what was sent down to them and that they might give thought
وَ مَا أَنْزَلْنَا عَلَيْكَ الْكِتَابَ إِلَّا لِتُبَيِّنَ لَهُمُ
الَّذِي اخْتَلَفُوا فِيهِ ...
And We have not revealed to you the Book, [O Muhammad], except for you
to make clear to them that wherein they have differed ...
and the household of the holy prophet PBUHH are among the people of the message, because the holy prophet PBUHH himself is a message قَدْ أَنْزَلَ اللَّهُ إِلَيْكُمْ ذِكْرًا . رَسُولًا يَتْلُو عَلَيْكُمْ آيَاتِ اللَّهِ مُبَيِّنَاتٍ and no one is closer to him than his household, PBUT, and because Allah AWJ has already stated that He has selected the household PBUT and only them to be clean of any impurity of any kind:
إِنَّمَا يُرِيدُ اللَّهُ لِيُذْهِبَ عَنْكُمُ الرِّجْسَ أَهْلَ
الْبَيْتِ وَ يُطَهِّرَكُمْ تَطْهِيرًا
Allah intends only to remove from you the impurity [of sin], O people
of the household, and to purify you with [extensive] purification
and so they are the only ones who can know all the secrets of Quran, hidden from others.
PART 2
Allah is the Wise, so any word that He has selected to reveal anywhere in Quran has a (or many) reason(s) behind this selection. The believers are always welcomed to think about the wisdom before such selection by Allah AWJ. Therefore your question is a good and valid question.
PART 3
According to Allameh Tabatabaei, the writer of the Al-Mizan interpretation of Quran, a great Shia scholar, among the miracles of Quran is that if somewhere Allah choose to talk through pronouns, any and precisely every possible interpretation of the pronouns which goes well with Arabic language rules would be valid. So it might be odd but even your simplistic interpretation of the pronoun "he" is valid, and this explains how a complete human being can be, as explained in several Ahadeeth. Beside that in Quran the holy prophet PBUHH is explained to be a witness over all his people in his era and future (and past in some interpretations):
فَكَيْفَ إِذَا جِئْنَا مِنْ كُلِّ أُمَّةٍ بِشَهِيدٍ وَ جِئْنَا بِكَ
عَلَيٰ هَؤُلَاءِ شَهِيدًا
So how [will it be] when We bring from every nation a witness [the
messenger sent to them], and we bring you, [O Muhammad] against these
[your people/messengers] as a witness?
and no one is allowed in Islam to witness what he has not himself observed, so that All-Hearing and All-Seeing are valid attributions for the holy prophet PBUHH at least when "All" is to denote all his people, or all the messengers.
