We've had a nice discussion,
Galin. It's made me think of going to see Warwick in the future...
I agree with your '
general, very simplified evolution' of Kortirion.
Despite, as you say, the lack of accuracy of some things in some of the poems, due to '
arguable whimsy', an '
incomplete knowledge of something historical', or an '
artistic take' on the same, there are still recognisable elements from actual events in these poems, such as 'The Hoard' being partly made out of elements of the tale of Túrin and Mîm the Dwarf.
At the risk of repeating myself, Tolkien
may have realised that, if he included this particular poem in the
Adventures, it would become 'canon', and necessitate a lot of work to fit it in to the 'accepted' narrative, as he had already done in revising 'Riddles in the Dark' in
The Hobbit.
But as you rightly said, all this is speculation, in the absence of further information.
