Clause 65.2 states that an assessment of a compensation event is not revised if a forecast upon which it is based is shown by later recorded information to have been wrong. The clause specifically relates to an assessment which in the ECC is the effect on Defined Cost plus Fee and the effect on planned Completion. The clause does not therefore encompass the notification of the compensation event which the PM has previously decided to accept.
Further clause 60.1(8) is the CE for the PM or Supervisor changing a decision which has previously communicated, so if the PM decides that the CE was not in fact a CE he is allowed to change his mind however would concede a CE in doing so. Note this is because clause 61.4 requires the PM to "decide" about the notified event whilst clauses 62.2 and 64.1 require the Contractor and PM to "assess" the effect of the event.
However, if the Contractor has incurred or will incur cost or delay as a result of this changed decision i.e. abortive costs, then these should be included in the CE assessment. It is possible that the Prices and Completion Date will be put back as they were before the CE but that is not a certainty.
To answer your other points: (2) if W1 its included in the contract then the Adjudication Table states the relevant timescales however if W2 is included then a dispute can be referred at any time; (3) if it got this far it is unlikely you would be able to put up a defence based on a mistake made by the PM. You either are or are not entitled to a CE under the contract, if you weren't and the only reason you recovered cost and time was due to a mistake then it is unlikely an adjudicator would help you to hold onto something you shouldn't have had in the first place, (4 & 5) there is no final account procedure in NEC3, the matter should be treated as another CE. PM should notify the CE as a changed decision under clause 61.1 and instruct the Contractor to submit a quotation, if the Contractor fails to do so the PM should assess it himself under clause 64.1.
Further clause 60.1(8) is the CE for the PM or Supervisor changing a decision which has previously communicated, so if the PM decides that the CE was not in fact a CE he is allowed to change his mind however would concede a CE in doing so. Note this is because clause 61.4 requires the PM to "decide" about the notified event whilst clauses 62.2 and 64.1 require the Contractor and PM to "assess" the effect of the event.
However, if the Contractor has incurred or will incur cost or delay as a result of this changed decision i.e. abortive costs, then these should be included in the CE assessment. It is possible that the Prices and Completion Date will be put back as they were before the CE but that is not a certainty.
To answer your other points: (2) if W1 its included in the contract then the Adjudication Table states the relevant timescales however if W2 is included then a dispute can be referred at any time; (3) if it got this far it is unlikely you would be able to put up a defence based on a mistake made by the PM. You either are or are not entitled to a CE under the contract, if you weren't and the only reason you recovered cost and time was due to a mistake then it is unlikely an adjudicator would help you to hold onto something you shouldn't have had in the first place, (4 & 5) there is no final account procedure in NEC3, the matter should be treated as another CE. PM should notify the CE as a changed decision under clause 61.1 and instruct the Contractor to submit a quotation, if the Contractor fails to do so the PM should assess it himself under clause 64.1.