These days, Glock is so well established that many shooters only want striker design and think anything else is just awful. You are quite right, through a moderate amount of practice it is possible to adapt to a different shooting system. Your brother in law's DAO P95 will be a good little shooter with some practice. Snap caps might be a good way to practice in addition to live fire.
The double action police revolver(Smith & Wesson and Colt) was very effective for over 100 years enjoyed one of the best safety records during that century plus of service. Safety is number one in my book and I feel that a firearm mishap is a more likely event than actual defense scenario, although both are possible. The DAO system is ultra safe as well as a very viable shooter. Seems like you can congratulate your brother in law for making a shrewd, well-informed purchase. He has a really good, safe and practical service pistol with the P95 DAO.