The regiment was indeed classed as a Warwickshire one till two years after Rorke's Drift, but I think its renaming recognised an existing situation, and although quite definitely mixed in its recruitment, it had a higher Welsh content than some people claim. Birmingham and its surrounding industrial areas were in a state of great expansion at the time, and many of its inhabitants came from Wales. The composition of British regiments is a complex matter, and nowadays hard to trace, but it is perfectly possible that one of the regiments recruiting in this conurbation could have been more attractive to the Welsh than others. There certainly seemed to be a high proportion of Welsh surnames.
Witt certainly claimed to have been at the Drift through the battle, and gave lectures on this basis, but his claim is rejected by Donald Morris in "The Washing of the Spears", which I think is still the best all-round book on the war and its background. The film has a tendency to inject far more interpersonal conflict than actually existed. I don't think there was that much difference of background between Chard and Bromhead, or that it would have cut much ice with the rest of the army if there were. Subaltern were hard on uppity aristocrats among their number. Certainly being a Royal Engineer wasn't seen as being second-rate or almost non-combatant. Gordon and Kitchener were both Engineers, and by 1879 Gordon was a legend with most of his faults largely unrevealed.
I certainly don't think there was much indecision about whether to fight. They knew they couldn't outrun Zulus, especially if they took the sick, and defending a small and fortified perimeter with modern weapons gave them a very good chance of survival. I don't think they felt incapable of further resistance if the Zulus had continued the assault, either, and their dominant feeling at the end, while not unmixed with horror, was probably that they had won. British regiments all have their different temperaments, and you could hardly have picked a better one to hold a good position without panic.
BluRay might make it easier to see that of the lines of Zulus on the hills above the Drift, many groups consisted of "men" of whom the middle ones had plumes and shields, but only those at the ends had legs. It was a low-budget film. I think the actual battle was pretty true to life, although the charges were probably more frequent, and certainly involved more than the 500 extras they had. One of only a few details of the fight I could fault is that I very much doubt if they held their fire until 100 yards. The range of the Martini was far greater, and as a stores depot they had plenty of ammunition. You don't get many shots with even a very fast-loading single-shot rifle in the time it takes Zulus to run 100 yards. It is a little surprising that more Zulus weren't killed, estimates mostly running around 500. But rapid fire would have resulted in the defenders firing into a much denser wall of smoke than is seen in the film.
I've just watched the extra material on my DVD version, including an interview with the widow of John Prebble the historian, who was one of the main scriptwriters. It seems that some of the quite incorrect characterisation of Hook etc., at which his family complains, came from Prebble, because he thought he had to dramatise a little more.
Colour-Sergeant Bourne was indeed 24, with little if any combat experience, and was already thought to be the man who was going somewhere. He became a lieutenant-colonel, as he might have done without Rorke's Drift, and died on VE Day. He recorded a radio talk of which no recording exists, but the transcript can be found online. He claimed that nobody was killed or seriously wounded by assegais, which is an exaggeration, as some men in the hospital almost certainly were. It seems to have been true, though, that most casualties were caused by gunfire, and the rifle and bayonet, in the hands of a man who knew not to engage it in the hide shield, was more than a match for the assegai. There is a tradition that King Chaka's substitution of the stabbing spear for javelins was inspired by the sight of the bayoneted musket, but the assegai might more correctly be classed as a mostly wooden sword. He is often thought to be wrong in saying that the Zulu firearms came from Isandhlwana, but the defenders heard the gunfire and saw the smoke, and I don't think the timespan makes it impossible.