The said is not sufficient in the establishing of any divorce, including a khula divorce; the religiously-prescribed formula (sighah) is necessary, in addition to the fulfillment of other conditions of such a divorce which are mentioned in the books of Islamic laws.
Simply desire to have it is not considered as insincerity, Riya, since Riya is when the action is with the intention of showing others. Having the prostration mark may or may not happen, so our advice is to ignore this issue and not focus on it.
The time in the said marriage is the right of the husband and only he can forgo it. But it is permissible for the wife to make a condition in the contract that she acts on his behalf in this matter. She can then forgo the remaining time.