Can wrist exercises help prevent carpal tunnel and other wrist conditions?

Not exercise per se. There are certain forms of therapy that can aid in preventing and treating carpal tunnel, but generally not exercise. In fact, for the most part it is important to avoid "overuse" or "repetitive trauma" to the wrist and underlying median nerve which happens with repeated or prolonged flexion of the wrist. But, various forms of deep massage at the volar wrist may be of some help along with...
Yes. Yoga has been shown to help decrease symptoms associated with carpal tunnel syndrome. Other exercises that result in striating the soft tissue structures and decreasing inflammation of the wrist can also help.