NEC ECC: Can a existing Risk Register item extend contract duration after completion cert issued?

Under Option A, a existing EWN on the Risk Register item has become a Compensation Event after the issue of the Completion Certificate (RR item was still defined as active, after completion). Can this CE retrospectively extend the Completion date, if notified in accordance with the contract?

As a compensation event can be notified up to the defects date then the answer is Yes. It doesn’t actually matter whether it is on the Risk Register or not as a CE notification is a separate matter, but be aware of the time bar issue under clause 61.3.