I have absolutely no issues with Khumalo picking up a few thousand a week (ie next to nothing, in football terms) if the club consider him to be a useful mentor for the youth lads. There is no way he'd consistently be our U21 captain if he wasn't great behind the scenes and guiding the youngsters, particularly the defenders. And I will completely trust the club's judgement on that!
Carter-Vickers looked very comfortable alongside him at CB last night. Playing alongside someone of his experience should help him develop nicely.
I can also only assume that Khumalo is content to have the role he has at the club, so it's a no lose situation really.