A question - if you're at 35ft, flying by instruments, is the PNF looking out of the window for obstacles?
I very much doubt it. More likely looking in trying to work out WTF is happening.
Obviously the problem manifested itself when there was still runway in front,
In my experience not very much. You are often into the last 900m of runway at JNB when you rotate, with a ground speed in excess of 180kts.
I know it's several seconds from idle to decent power, what is it from typical flex t/o power to TOGA?
If there is any derate at JNB it's usually pretty small. I'd be surprised if it took more than 5 secs to go from derate to max power.