For (1), there would probably have to be a change in the Works Information leading to a compensation event ... I can't think of any other alternative under the contract. The only other recourse would be to go down the misrepresentation route under contract law if you believe they deliberately (which is fraud & a criminal offence) or innocently (a civil transgression) misrepresented the situation to you.
For (2), the strict answer is 'No' except by agreement between the Parties i.e. Employer and Contractor, not the PM and Contractor and a supplementary agreement. However, under option B, lump sums are paid per proportion complete so there is nothing to stop the PM and Contractor breaking it down further to reflect the proportions for working purposes.
For (2), the strict answer is 'No' except by agreement between the Parties i.e. Employer and Contractor, not the PM and Contractor and a supplementary agreement. However, under option B, lump sums are paid per proportion complete so there is nothing to stop the PM and Contractor breaking it down further to reflect the proportions for working purposes.