Also, the Brits did not adopt the FN-FAL until the late 1950s, I believe, and it was probably a few more years before the new rifle was completely distributed to all soldiers.

So if we suppose that surplus is not made available until after a rifle is obsolete, this would put us in the early 60s, right? Do we have evidence of Lee Enfields arriving in the USAicon before that?

The surplus story is a valid part of the Lee Enfield Story. Consider two things:
1) The Lee Enfield has had a surplus life (c1960-2009) that is almost as long as its official service life (1888-c1957).

2) The Lee Enfield has probably seen as many (or more?) rounds fired by civilian as by soldiers, especially if you consider civilian target competition and worldwide hunting use.