NEC ECC: Recourse after CE implemented

Under option A, is it possible for the Contractor to challenge the cost of a compensation event after it has been implemented by the PM?

Only contractually via adjudication.

Practically you can explain why you disagree - and only through agreement would they ever reopen it. Also would be better for PM to share there assessment to the Contractor to give them a final chance to justify why they think it should be more cost/time before PM finally implements.