Do they essay graders take into account whether one is a foreign applicant or American in grading the essay? Also can a good essay offset worried of a low Verbal score?

The graders do not grade differently based on the test taker's origin. I don't know for certain, but I would be highly surprised if any school used the essay grade to offset a poor verbal score.

As a general rule, the schools do not care as much about the essays as they do about the multiple choice sections. If you score a 4.5 or higher on the essay, you'll be fine. If you score below that, the school might be concerned, particularly if you score very poorly on the essay portion but turn in fantastic essays with your application. Then, they might think you have had someone else do your application essays.