[gottago]

Aren't the two extra contestants essentially standbys in case someone pulls out early.

No they do have an extra standby. Jordan North was a standby originally but another contestant pulled out quite early in the 'hiding' process before the series and he went in instead. Ruthie and Russell remained the two latecomers.

I can't see them replacing Olivia at this point as the two latecomers will dominate the storylines soon and her leaving and needing to be replaced be a bit of a distant memory in the nicest possible way. If she'd have left after they came in then they probably would have sent the standby in.