I don't think magistrates rule on questions of law (maybe you were implying this, but maybe not). But in general the whole legal/justice system is basically blind to the harm it itself causes, so I don't think an actual judge looking at the merits of a warrant would be terribly adversarial to a sheriff either - they work together all the time, and most of the warrants presented by the sheriff are legitimate.

I do agree with you in general that we should aim to split system functions between multiple people. But this merely raises the bar, it doesn't make corrupt actions impossible. Which means we should be focusing on both avenues of reform, rather than emphasizing one to downplay another. Especially as when you do this, the entrenched system seems to takes advantage of the downplaying while resisting the solution being emphasized.