I have a vague recollection (although I cannot back it up with a newspaper article), that Roux offered Nel CH but Nel turned it down and wanted Premeditated Murder which meant Roux/OP had to come up with a defence to murder. They then tried PPD but OP messed up on the stand. This was not discussed in court and may well be that it was just a rumour. I agree Roux never actually said OP was prepared to plead guilty to CH.
It would have been open to the defence to plead to CH when OP entered his plea
Of course there is no real reason for the defence to do so when they are running a PPD defence