Is it permissible to adopt the Maliki opinion regarding the purity of water that has separated from an impurity before the impurity itself was purified, in order to avoid obsessive doubts (waswasa)?

It is permissible for a lay Muslim to follow the Maliki school of thought concerning the aforementioned issue. A lay Muslim is allowed to follow any of the recognized schools of thought of the people of knowledge, and it is not obligatory for him to adhere to a specific school. Ibn Hajar al-Haytami stated in "Tuhfat al-Muhtaj" that it is permissible to follow any of the four Imams, as well as others whose schools of thought have been preserved and documented.