According to Agha Sistani, can the divorce of Khula be reversed with mutual consent of estranged husband and wife through some means – let's say the husband returns the money to the divorced wife and convert the divorce into a simple divorce where the two can marry immediately? 
And does this have any time limit to it, Agha, meaning the divorce cannot be reversed after passage of certain time period?

If she accepted returning the money within the Edda period, then it automatically becomes simple divorce, and he can return her to the marriage without a need for another contract.
However, after the Edda period, they can start new contract, with no need to return the money, but he to pay new dowry.