How should a Muslim deal with the widespread prevalence of Sharia violations in all spheres of life in Islamic countries? Should he participate and attempt reform, or withdraw and allow corruption to spread?

It is absolutely impermissible to work in usurious banks, as no benefit is realized or harm averted through such work. As for working in the police force, it involves significant benefits, such as aiding the oppressed and deterring the oppressor, while still necessitating caution against assisting the unjust.