I agree there is something of a mystery around Isnadlwana, and like you, that caught me. I read quite a bit, clearly, nowhere near as much as some. Once your head is full of stuff then you look at detail and check information,fill in the detail, check sources, motives etc, no I am not a copper.
It does not take too long to realise that the contemporary politics around this Battle were poisonous, that has consequences.
So I start from a position of thinking about what information I would expect to see, and what I do see, and then ask why am I not seeing this. Much recorded stuff that we do see is written in such a way as to appear to say something but actually is quite ambiguous.
I agree there are inexplicably "missing pages" pronbably as I see it because some people figuratively tore them from history, of failed to ask or answer questions. The why is the poisonous politics.
One example is the famous order to "Defend the camp" apparently written/added by Clery almost as an after thought, (I forget where I read that) not that that is material to my point, but if true material in itself for other reasons.
What is meant by camp? The camp was at least a rectangle of 800 x 300 yards, plus an area on the nek and ground which held the wagon train all 300 of them and the 1500 or so oxen to pull them. It is fanciful to think that there were no stores on the wagons, the oxen may be outspanned but the wagons were essential stores so had to be defended as well. The dispositions of the camp ensured that it was indefensible. The Nek was not defended because Chelmsford had convincedhimself that there were not Zulus behind him and none couold get there in force without him knowing and stopping them. For sure if all combatants had stood shoulder to shoulder they may well have survived, but the "camp" would have been lost. Neither Pulleine nor Durnford had any chance to "Defend the camp". Why? Because the Zulu executed a good plan perfectly, and got lucky due to the missteps of the British leadership.
Ntshingswayo was never asked about his opinion as to strategy.
The court of inquiry left much unasked.
I have reached the conclusion along with others I think that the answer in much more prosaic. The Zulu on the day had the better plan and executed it to perfection. On the British side there were careers and reputations to protect, there was also the reputation of the Army to protect, prestige was very important.
The missing pages may be out there to answer all the missing details, I doubt they are. But the fact remains on the day the Zulu destroyed the camp, because on the days of the campaign they were the army that performed its function at the level required to destroy the opposition. That was no mean feat.