Shunderson

It is possible, and has been known to happen.
Contrary to what we see in many westerns the process of state-held hangings was to break the condemed's neck.
If the hang-man was not careful and accurate in calculating the drop, people could survive the drop. If a burly man with a thick neck survived the drop he could be asphyxiated to the point of unconsciousness, which may seem (again to the un-carefull executioner) as being dead, and could be "cut" down before actual death.

Now there should have been a doctor to confirm the stopping of the heart with a stethescope - but it is conceivable that the doctor heard what he expected. It was said that it was raining, so everyone involved could have been in a hurry and sloppy about it.

I believe his slow wittedness was being attributed to the hanging.

So, everyone was fine with Pretorias having a big, menacing bodyguard, who was a double-murderer?

Paintings are hung. Criminals are hanged.

How do you know he was hung?
He may have been but, after being convicted of murder for the second time, he was hanged.