It sounds a mess!
If the standard form has not been amended by Z clause(s) then the default assessment of compensation events remains as Defined Cost (the change in Defined Cost due to the CE) + Fee.
You should however be mindful that the use of bill rates for assessing CEs is only by agreement and therefore the PM may not agree to the use of rates for omissions (which I assume are CEs reducing the Scope).
If the standard form has not been amended by Z clause(s) then the default assessment of compensation events remains as Defined Cost (the change in Defined Cost due to the CE) + Fee.
You should however be mindful that the use of bill rates for assessing CEs is only by agreement and therefore the PM may not agree to the use of rates for omissions (which I assume are CEs reducing the Scope).