I have assumed that you are using an Option C contract.
Under the standard contract the Contractor is entitled to payment of the Price for Work Done to Date even if it has not achieved Completion by the Completion Date. The initial assessment of the Contractor’s share being applied at Completion.
Option X7 is used as a pre-agreed rate of damages for the Employer to recover if the Contractor does not achieve Completion by the Completion Date. X7 does not affect the PWDD but is considered in the assessment of the amount due under clause 50.
Yes, the Contractor can notify of CEs after the Completion Date obviously subject to the provisions of clause 61. Please note, the accepted programme does not set the Completion Date, the Completion Date is managed under the CE process (except for specific circumstances).