Dear Jardee ,
As per the contractual condition that you states in the scenario , this event shall be started after the final approval form the client side for the change order , however the contractor started the work before this approval date , so you can use the contractor start date as the event start date and make the analysis after this date to get your entitlement.
By the way form our point of view the contractor has all the rights and entitlements as per the contract for any delays till the approval date for the change order
I hope that my answer will be clear for you , and if you have any further questions , please don’t hesitate to post it again.
Best Regards,
Mahmoud Omara