How close is the published The Silmarillion to JRRT's Silmarillion?

[Galin]

Hmm, well we have this from CJRT for example, in HME itself...

'The Silmarillion', again in the widest sense, is very evidently a literary entity of a singular nature. I would say it can only be defined in terms of its history; and that history is with this book largely completed (...). It is indeed the only 'completion' possible, because it was always 'in progress'; the published work is not in any way a completion, but a construction devised out of the existing materials.'

'Those materials are now made available, save only in a few details and in the matter of Túrin just mentioned; and with them a criticism of the 'constructed' Silmarillion becomes possible. I shall not enter into that question; although it will be apparent in this book that there are aspects of the work that I view with regret.'

Christopher Tolkien, The War of the Jewels


Of course, then there is the interpretation of that much