Please see the attached graphic image, comprised of Virginia land grants on Staunton River. I have identified the location Mush Island for you, on this map. It was on the river west of the Halifax line, and east of Reed Creek, which is also marked. Note the abundance of Patey / Pattey and Doss grants in this vicinity. The John Denny grant of interest is near the center of the image.
I trust this should answer the mystery question regarding the location of Mush Island on Staunton River. It does not appear on modern maps, suggesting it was washed out in a flood at some time after the start of the 19th century.