It’s similar to searching for consistencies amongst any mythology, which is what Tolkien was attempting to create. Tales will always change over time, and they’ll always shift focus to what the teller determines is important. As focuses of a society shift, so do the focuses of its related mythology. In this way, I think Tolkien did an excellent job creating a united mythos for England in all the different versions of his legendarium. As the tales evolved, consistencies emerged elements which were formerly key, were discarded, and internal references became more commonplace than external references (see Tolkien’s influences from William Morris and Icelandic, Celtic, Germanic, and Anglo Saxon epics)
It’s been a few years since I’ve read the foreword to the Silmarillion, but I’m glad I’m consistent with Christopher’s analysis 😁