Question about the Manton Limit from an outsider

Newcomer to the series here, is the Manton Limit basically a tool to ensure everyone doesn't kill each other simultaneously due to power abuse?

While watching Across the Spider-Verse I've always wondered why Spot can't simply portal into a person's head and pluck their brains out. Is the Manton Limit supposed to explain it?

Also, side question - can't the Manton Limit be expanded to create conventional nullification defenses against Parahumans? Or is that too advanced for the story?