A few times I recall Andy being referred to as the "High Sheriff." Was this related to Mayberry being the county seat of Mayberry County? Was Andy actually the head sheriff of the whole county?

Speaking of which, Mayberry didn't seem big enough to be a county seat.

Good question.

Andy once stated that they were paid by the county. The county was Mayberry County. But Mount Pilot (which in reality is a real mountain and not a town) was a larger community, so Mt. Pilot to me is the county seat. So the county could have been named "Mayberry County" while the county seat was Mount Pilot. But then again maybe Mayberry itself was the county seat of the county,. but this was never explained.