Dear Andrew,
For one thing, I think by the time he wrote PF, VN knew that he was an artist of an entirely higher order than RLS.
Do you have any evidence that he changed his mind? I don't think this has to do with who is the higher order of artist, but we know that VN was both fond of RLS and had a higher opinion of the story of J&H than you do.
One¹s muse can be angelic, and one¹s muse can be demonic.
The versipel is not just any demon, it is a changeling. Do you really think that VN chose this very odd word - - which doesn't even exist in most dictionaries - - because he needed a three syllable word meaning "demon"? He could have chosen from such words as Azazel, Ahriman, Mephisto, Apollyon, incubus, succubus, diablotin, poltergeist, devilkin, erlkönig, hobgoblin etc. Just a few from my Thesaurus. It seems to me that erlkönig would suit your interpretation very well and has the added value of having been referred to in the poem.
So why "versipel"? Why is Shade's muse a changeling?
Carolyn