With the understanding that you've based this class upon the television series and not upon the (IMHO vastly superior) books, I greatly suggest reading the books. There are a number of aspects in Goodkind's writing that were ignored or outright bastardized for the television series, and these aspects could help bring balance to the class.

If anyone wishes to avoid spoilers, I've made the following comments hidden. Read at your own risk of spoilers to the book series.
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First and foremost is the way a Mord-Sith's connection with magic works. They are not immune to magic. What they do, however, is bend a person's magic back on them. If you use your magic on a Mord-Sith who is not bonded, she may take your magic and turn it against you. She doesn't control the magic, she simply denies you the use of your own magic and causes you pain through it should you try to use it. A Mord-Sith cannot be bonded to more than one person at a time.

Breaking someone is not an easy task, either. It is much more the result of psychological manipulation than pain, by the agiel or not.

Plus, an agiel is painful to the touch, even if one is just hold it. A Mord-Sith's pain might make her angry, but the agiel doesn't feed off of pain. It's actually a very precise instrument.

With that in mind, I would suggest making the Mord-Sith PrC focused on learning to use the agiel better. Each level offers a new technique, and perhaps the ability to shrug off the nonlethal damage it should be doing to it's wielder. In the hands of a non-Mord-Sith, the agiel is a fairly crude tool and an even cruder weapon.