Yes, the permission of his wife is required for it to be permissible for him to marry her niece. The man does not need the wife's consent to marry her aunt. There is no difference in the two scenarios between the two types of marriages.
Wearing the white turban is to show that the person wearing it is not related in lineage to the Prophet (peace be upon him and his holy progeny). Likewise wearing the black turban is to show that the person wearing it is related to him in lineage. There is no social discrimination in such a thing. However, one can understand from some rulings that have been legislated the merit of being related to the Prophet, such as the impermissibility of the Sayyids to take charity from non-Sayyids, and the like. As such, it is good to respect the Sayyids due to their genealogical relationship to the Prophet and his Ahlulbait. This, again, is not social discrimination, as it is only in order to strengthen the respect of the Prophet in the hearts of the believers. So, just as it is recommended to recite the Salawat on the Prophet whenever his name is mentioned, it is also recommended to respect and venerate his offspring. In doing so, the Great Prophet and his Ahlulbait (peace be upon them all) will remain respected and revered in the hearts of the believers. Having said this, it does not mean that whoever is related to the Prophet and his Ahlulbait will not have his deeds accounted for; rather this increases his responsibility in abiding to good deeds and the truth, and avoiding sins.