I understand your issue here. The first point is that yes you are entitled to include within any CE quotation risk which has a significant risk of occurring and is your risk under the contract. If the PM is willing to give you an assumption under 61.6 as to what you should assume then it would take the risk away from the Contractor as this element of the implemented quotation would be subject to remeasure.
However without any such assumption then the Contractor has to consider and include such risk within their quotation. As you allude to - difficult to second guess but like any risk you have to give it your best stab. Too much and the quote is unlikely to be agreed and they would make their own assessment (using their best stab) which could then only be challenged (officially anyway) in adjudication.
Difficult to do, but then no worse than what you had to do at tender stage for the whole job!
However without any such assumption then the Contractor has to consider and include such risk within their quotation. As you allude to - difficult to second guess but like any risk you have to give it your best stab. Too much and the quote is unlikely to be agreed and they would make their own assessment (using their best stab) which could then only be challenged (officially anyway) in adjudication.
Difficult to do, but then no worse than what you had to do at tender stage for the whole job!