"As is seen in The Silmarillion. This is not an Eldarin title or work. It is a compilation, probably made in Numenor, which includes (in prose) the four great tales or lays of the heroes of the Atani, of which "The Children of Hurin" was probably composed already in Beleriand in the First Age, but necessarily is preceded by an account of Feanor and his making of the Silmarils. All however are "Mannish" works." JRRT, The Shibboleth of Feanor, note 17, The
Peoples of
Middle-Earth.
It won't surprise you that I restate my dim view on JRRT's later writings.
Things being compiled in
Númenor (most likely early in its history) are all very fine. But he babbles (I can't use a less harsh term here) about "
four great tales", the fourth presumably being the "Lay of Eärendel" or something of the sort, supposedly the longest of the four. What exists for it is perhaps a page or so of scribbling which make it clear to me that JRRT would have been
incapable of writing it. Before Elwing appeared with the Silmaril, Eärendel is supposed to have explored a good part of the planet - even have dispatched of
Ungoliant in some obscure squiggle. This would have taken Eärendel to
places outside of the M-e JRRT had described up to that time. Forget it.
And how are works compiled in Númenor supposed to have survived its downfall? "The shadow falls on Númenor" is 1800
Second Age. 2251 we then have "Rebellion and division of the
Númenóreans begins". 1,004 years later
Ar-Pharazôn seizes the scepter, 7 year later A-P takes
Sauron "prisoner". One thing all authoritarian / totalitarian rulers have in common is a concerted effort to destroy historical documents that make them look bad. Sauron had round about half a century in Númenor in which to go at this task.
There is one feasible line of transmission, starting in the year 600 SA when the first
Númenórean ships appeared on the coasts on M-e. One place they would have frequented were
the Grey Havens, ruled by
Gil-galad in the SA,
Cirdan afterwards. Later in Númenórean history only
the Faithful came to these ports.
Elrond lived here too, before he retreated to (what was to become)
Rivendell in 1697 SA before Sauron's onslaught on Eregion, then almost all of
Eriador. After Sauron got his backside kicked out of Eriador in 1701 SA, Elrond had 1,700 years
in that age to view and edit any garbled "mannish traditions" that may have been collected in Rivendell - and never mind roughly 3,000 years of
the Third Age. He would have preserved them - I'm thinking first edition of TH here - but they would have had annotations in the sense of "ah ... no ..." attached to them.
JRRT's "mannish tradition for the Silmarillion" is one of his weakest ideas in the legendarium.