More or less correct. The last attacks were in the early hours of the morning, and dawn is around 05h15 - 05h30 in Kwa Zulu Natal at that time of year. It's unlikely that the defenders realised the enemy had withdrawn until about 06h00 and, as far as we know, the British left their positions at this point and started searching the area. The Zulus reappeared about an hour later when and they retreated behind their barricades again. Chelmsford and his men arrived around 08h00 and probably took part in the mopping up operation and burial of the dead.
Various reasons for killing the wounded have been put forward, but I don't think there is any single, and compelling, explanation. It may have been a combination of post battle stress, revenge for the killing of the sick and wounded during the attack on the hospital, and concerns about leaving wounded men in the vicinity of the mission station who could still have posed a threat, particularly if there was another assault. It's also been suggested that the defenders were outraged by the Zulus ruthlessness and mutilation of the dead at Isandhlwana - in accordance with their own customs - but this is speculative. We just don't know.
FWIW, Rorkes Drift is an interesting place to visit. It's remote, and hasn't really changed a great deal since 1879. The original church and homestead/hospital were pulled down after the battle, but their replacements are in more or less the same positions and it's easy to reconstruct the events. Isandhlwana is haunting.