Claiming more than the mahr amount in khula'

My wife is seeking khula/divorce on the ground of alleged "irrevocable differences". I agree (reluctantly) provided she reimburses my mahr as well as other items of wedding and walima dresses and suites and other items of jewelleries. I have been waiting for the past six months but her (parents) have been non cooperative and belligerent in a sense, NO RESPONSE at all.

Initially they went to Islamic Sharia Council in London, UK and petitioned for divorce which I contested on the grounds that the cause of marriage was her (fraud on the grounds of illicit friendship outside our marriage) entirely her own fault. After a couple of months they decided to withdraw her petition from Islamic Sharia Council. She has now enlisted the help from Birmingham Central Mosque citing the same false accusations and demanding divorce. I will strongly contest on the same grounds. I will give her khula on condition that meets my certain conditions like the return of my mahr, my jewelleries and expensive items of clothing’s. From Islamic Sharia prospective am I right in my demand of my goods?

In a nutshell she wants a divorce from me because I CAUGHT HER RED HANDED with a "family friend" (not even a family friend but just a friend. She is not prepared to break up their friendhip for the sake of our marriage). Please advise me on where I stand.