Can the PM issue a CE adding more work (changing the works information) after Completion, but before the defects date as per cl61.7?

Yes!

Although he would at that point have little control as to how long the Contractor could take (as there is no recourse for over-run as delay damages are redundant after Completion), and the Defect Date would not be changed.

In these type of situations it would be better to have a supplemental agreement agreed between the Parties