NEC ECC: Does a Compensation Event form part of the Works Information?

And if the CE was issued after Completion, does the Completion Date have to be revised to after the additional works?

A change in the Works Information is a compensation event (subject to the 2 bullet points of clause 60.1(1) but not all compensation events form part of the works Information eg a CE due to weather measurements.

Completion and the Completion Date are 2 different matters. If Completion has been certified before the CE being issued then the work covered by the CE was not required to be completed by the Completion Date. Hence the Completion Date does not need to be adjusted. Please also note that adjustment of the Completion Date is based on the movement in planned Completion due to a CE not the actual timing of the CE ie if the Contractor was in a delay of his own making he would not recover that delay.