which muslim ruler ordered the construction of a magnificent mosque in istanbul and decreed a vast new set of laws for his empire
The answer is: Suleiman
Suleiman was regarded as the greatest Sultan in the Ottoman empire. He constructed a magnificent Mosque as a praise to Allah that he credited to for the prosperity that happen to his kingdom.
Under his rule, he created new set of laws that regulates education, social treatment, taxation, and criminal punishment which harmonized the relationships between different groups in ottoman empire.