I have assumed that you are on either an Option C or D contract.
It is one of the PM’s actions to assess the amount due (assess an application) under clause 50.1. so the answer to the first part of your question is yes.
In respect of disallowed costs, these are defined under clause 11.2(25). Resources not used to Provide the Works (after allowing for reasonable availability and utilisation) is a situation for disallowed cost.
The PM does not incur disallowed cost so I have understood your second question to be that the PM is including a forecast of disallowed cost to be incurred by the Contractor.
The PM considers any disallowed cost when making his/her assessment however the disallowed costs can only relate to the Defined Cost that he/she is including within the assessment (prior to their deduction).
Therefore, no the PM is not entitled to include a forecast of disallowed cost unless he/she has also used the same amount within his/her assessment of Defined Cost.
Does the above help?