[Mithadan]

This post is made at the risk of disappearing down the rabbit hole of "canonicity."

Myths Transformed is a fascinating section of HoME. It gives insight into Tolkien's creative process and his self-doubt regarding how previously well-established portions of his mythos fit within the whole as well as with the Primary World. That said, Myths Transformed must be taken with a grain of salt. Much of the material presented is speculative and their is no indication that it represents JRT's final formulation of how the Silmarillion might be rewritten despite the late date of the writings. Much of the material presented represents a significant departure from his prior conceptions and simply do not align with the Mythos. By way of examples, his "round earth" version of the history of Middle Earth leaves unexplained whether the tale of the Two Trees can be preserved (placing in doubt the origin and nature of the Silmarils themselves). The recasting of Orcs as corrupted men rather than Elves would require a significant revision of the time table of the First Age. His musings regarding whether the tales can be portrayed as a mannish (read Numenorean) distortion of Elvish legend would have significantly impacted the "believability" of the Mythos, and its presentation as a history. As commented above, the "execution" of Morgoth is inconsistent with JRRT's conceptions of the nature of the Valar and their nature.

The observation that the Prophecy of Mandos aligns with the biblical version of apocalypse is an apt observation, but draws into question Tolkien's professed dislike of allegory.

Just as another observation, the Prophecy of Mandos may also be affiliated with the very early legend of the Elvish "Faring Forth" found in Lost Tales. The relationship is not direct, but the concepts seem to potentially be linked.