The project manager is able to issue instructions and notify them as compensation events up to the defects date, see clause 61.7. So, in answer to your second question, you have to comply with those instructions and cannot refuse to carry them out, see clause 27.3.
The compensation events for the instructions are assessed in exactly the same way as any other compensation event, that is based upon the forecast change in defined cost of carrying out the works that they cause, see clause 63.1.
It is not clear what you mean by ‘remobilisation costs’. If those forecast defined costs fall within the definition of defined cost, see clause 11.2(22), and the rules for calculating them are in the shorter schedule of cost components, then they are allowed in the assessment. Such costs are likely to be higher but that higher cost is allowed in principle.
For example, if you need a member of staff to visit site to supervise them then the defined cost will be increased and therefore allowed under people, whereas if that member of staff was already on site during the project, it would not have been.