Hello CuriousQuestion,
A similar question has been asked and answered here which may help.
NEC4 Option A - Quote substantiation - NEC3 and NEC4 Contracts - ReachBack (builtintelligence.com)
Your request in my view is not unreasonable. It appears from your question that you are aware the Notification of a Compensation Event has been raised after the event has occurred, as well as the impact, Therefore, the Dividing Date proceeds the impact. You are asking the Contractor to substantiate ‘the actual Defined Cost of the work done by the dividing date’.
The most logical way of substantiating the actual Defined Cost would be via invoices / timesheets / Site Diaries. Other methods could be Purchase Orders or Subcontractor quotations.
What may help the answer to this question is how the Contractor proposes to substantiate their quotaiton.