Who is Buried in Justice Johnson's Tomb?:
Not Johnson, it seems. Perusing the strangely interesting Google Maps site on where deceased Justices of the Supreme Court are buried, I came across this entry for Justice William Johnson, an Associate Justice from 1804 to 1834:
Justice Johnson's interment whereabouts are unknown. Although there is a memorial dedicated to him in St. Philip's Church in Charleston, S.C., there are no records indicating that his remains are actually there. After his death in 1834, his remains were supposed to be transported from New York to South Carolina, but they never arrived.
Maybe somebody should have filed a habeas corpus petition.

  Thanks to How Appealing for the link.