Q1Assuming the work will be done after the contract date and assuming you are not changing the definition of the Price for Work Done to Date for the work to be done prior to the access date, you do not need to formalise the agreement. In summary for Option C or D the PWDD is the Defined Cost plus fee. Defined Cost is based on the Schedule of Cost Components for cost incurred in order to Provide the Works; there is no time stipulation (if after the contract date). For People they will need to comply with the Working Areas requirement.
If the definition of PWDD is being amended eg you are on an Option B but the Client has agreed to pay Defined Cost and not work x rates, the you will need to confirm the agreement in accordance with clause 12.3. The agreement will need careful drafting to ensure it works.
Q2 From a legal perspective, there is a risk that you do not have permission to be “on site”.