"Fascinating & a Big Mystery."
If that refers to the mystery of what happened to Doc Nixon, I found much of the answer and revealed that research in 2009.
William J. Nixon, (aka Doc Nixon & others) died in New York City on December 13, 1945, at Goldwater Hospital in Manhattan. He was later buried, (in an unmarked grave) in Brooklyn, his birthplace and early home.
His income records show no employment from 1937-1039, and just sporatic, occasional earnings around Manhattan, until his death in 1945, where he was living in a building next to a Catholic Church and School, in Manhattan's Upper East Side.
While there is the (unproven) possiblity he may have been involved with a Church group or Order in St. Louis or elsewhere, there is no evidence (yet) of him taking any vows or having a religious vocation of any kind.
There is no proof that Nixon ever left the country after 1937, (no passport or ship records) and if he did, certainly did not stay there.
As I noted in 2009, if anyone back then had actually tried LOOKING for Mr. Nixon, they could have found him in the Manhattan telephone directory with his fellow New Yorkers.