For options C/D/E&F, the cost of correcting a defect AFTER Completion is a disallowed cost, therefore Contractor risk. Correcting defects before Completion therefore under option C that you specifically mention is not a disallowed cost. Any associated costs will be delay with under the “pain/gain” mechanism depending if they are under or over the target price. If they are under target, the cost of correcting defects will be payable but will not increase the target. On a 50/50 split the Contractor is in effect paying for half the cost of correcting defects as is the Employer. It is not therefore in the Contractors interest to do defects as it costs them gain share which would have otherwise been pure profit to them. It does also mean that if any exist they should correct them before Completion - which suits the Employer as they are more likely to get a defect free job handed over.
Whilst an Employer may feel aggrieved at having to pay anything for correcting defects they should remember this is the shared risk nature of option C contracts. The Contractor would not get paid for them under option A, but then would have more likely included more risk in their tender price for that fact.