One other issue that must be considered is that the guvnor's overiding concern was with his invented languages.

Tolkien felt that a language could not be effectively constructed in isolation, that is to say without reference to history/mythology etc. I direct the interested reader to the short essay "A secret vice". Also, if my memory serves me, I believe there is mention of this idea in "Letters".

A fundemental reason for the construction of the Silmarillion in all it's phases was to provide a historical background to the development of the elvish languages.

Thus we have the thread of the intertwined threads of mythology and language commencing with BoLT through the Quenta, being combined with the Hobbit in the mid to late thirties and subsequently developed in the LoTR up until the mid fifties. Everything after this period appears to be a (somewhat futile IMO) attempt at rationalising what had become an incredibly complex weave of story and linguistic theory.

Of course everybody has a different idea of what can be considered canonical, but for my part I prefer to think that we can only strictly include work published during Tolkien's lifetime as being his final thoughts, the man after all apparently changed his mind about what we would imagine to be some of the most important events at each revision. Everything else is speculation, but this is another matter for discussion entirely.