The following is PURE SPECULATION:
I suspect they had mortar, since Rorke's Drift was a forward supply depot and such repair things would be needed for various projects. Mortar may have been for some kind of bridge supports that Chard was working on before the battle.
I don't think the holes were carved, but probably either laid with flat stones or lined with cut boards. In some of the spaces, you can see small straight rocks lining the holes. The holes are also not perfectly square, but are square-shaped, with different angles and sizes of holes.
The interesting hole/port is the one on the extreme left, which is higher than the others and has a stone step in front of it. I suspect it was an after-thought, or perhaps there is some obstruction (such as the stairs leading to the upstairs door) that required it be a bit higher.
Another interesting thing is on the top of the wall, it has parapets, but is lined with mealie bags (probably to rest the rifles on).
As to the speed of construction, I understand the relief column had well over 1,000 men, of which many could be put to work on the projects.