Regarding the Silverthornes in Greenville, I'd like to add that Spencer was the fifth of six children born to his father, Daniel, and his first wife, Elizabeth. His mother died when Spencer was four, his father remarried and they had six more children - Twelve Silverthorne offspring total (albeit one of whom died in infancy).
His birth siblings seem to have scattered about the country as Spencer did, but his father, stepmother, and most of his half-siblings remained in Michigan. I would imagine the current Silverthorne residents are of some relation to Spencer.
Spencer's first wife, Beulah Fowler was also from Greenville (she died a year or two after the Titanic sinking, their marriage produced no children). I haven't checked but you may find some of her relatives there as well.