Notice all disputes are in fact, about Quran and Sunnah and how to refer to it or who the authorities are and regarding their commands (in case of Shiites, who Ulil-Amr are is division Ismailis vs twelver vs others).
Quran and Sunnah can't solve disputes in themselves. In Nahjul balagha, it's said to mean Quran and Sunnah, but in tufuqul uqool the exact same letter (as that one) has alternative explanation and words to 4:59 and is in more in line with another report from Imam Hussain.
We need a book from God + Messenger from God to solve disputes. All misunderstanding occurs when role of Nabi and Messenger has been inverted. It's the opposite than what Sunnis say about these words.
A Nabi is one with a book and Messenger means he conveys the clear truth in his own words and he does the conveying.
The reason why it didn't say Ulil-Amr is because if disputes occur, it's not sufficient to get them solved by their roles as Ulil-Amr but rather you need a Messenger. In the case of disputes which other verses warn not to get into, but in case, you get into them, you need a Messenger to clarify them and reunite people on the truth.
That is one reason, another, is that the last Ulil-Amr would go into occultation, and it would not be possible to refer to him in public, public can't gather around him for a long period - till he comes back that is known.
The Ulil-Amr were not predestined to become Messengers, but if disputes would occur, and divisions and darkness covers the truth, and the clear signs of Quran begin to hidden and darkness of sorcery covers it's clear recitation and flow, Allah (swt) would assign them messages they are to paraphrase in their own words and convey.
So they did end up becoming Messengers...
And so this solves the confusion regarding this. That, and look at the context of preceding verses linking the issue Authority with the great authority of the family of Abraham.
So it's obviously the family of Mohammad or the comparison would not make sense.
Also, 4:83 is very calculative. It says what would've happened, it's not a command. It's saying if you referred to these people along with the Messenger, this and that would happen, meaning, the Ulil-Amr are sufficient as guides to matters of safety and fear.
The minhum doesn't refer to Ulil-Amr but the people meant to refer to them. You got to start seeing alternative interpretations that first impressions.
The reason is quite simple. IT's because there is a sorcery on hearts and minds of humans, and put's locks and ties knots blown upon with respect to Quranic clear recitation.
The Quran, the more knowledge you get, the more profound it becomes, but also the more simple.
This is the Creator watching our divisions saying 4:59. Think about the divisions we have, and think about the verses after, which show they have to come to a living Messenger.
There is a huge difference in going to your scholars who tell you the Sunnah is and their interpretation of Quran and Sunnah, and their judgment, than the Messenger.
Disputes - they won't be solved by scholars claiming to be scholars and authorities of Quran and Sunnah. It will be solved by the Mahdi, who will be sent (like ahadith say, they use "will send") by God with signs (miracles) + a message to revive the Quran and Sunnah.
The only thing the Mahdi is not a Nabi and so Quran is the last revelation humanity receives through a channel (which is a Nabi is defined to be in Quran).