Dear Jansy,
What disturbs me about your hypothesis concerning Kinbote and Shade as "one" came to my attention only quite recently. If Kinbote is a projection of split off aspects that belong to Shade ( the division could follow other strands, as you also suggested when connecting them with Stevenson's J&H ) then there would have been no Kinbote outside upsetting the lids of trash-cans and rapping against the window.
Kinbote does not literally upset trash cans or make jokes with professors over lunch. These are metaphors for his attempts to make his presence known to Shade. Remember that his first appearance in America coincides with Shade's illness ( a heart attack? a stroke?). His emergence as a separate fully conscious character does not occur until the stroke(s) that turn Shade into "a tipsy witch" and Shade's subsequent "death." Physically of course he is not separate.
In that case, John Shade would also be the seducing Erlkönig who carried Hazel away to her death ( following the hints in Goethe's poem).
I'm not sure what you mean by this, but I do think that Shade was responsible for driving Hazel and "the other one in ballerina black" to suicide. Does that qualify as a seducing Erlkönig? I don't know.
Kinbote would also necessarily represent Shade's repressed homosexuality. Does this appear Nabokovian to you?
Kinbote does represent Shade's repressed homosexuality. Do you mean this is too "Freudian" to be Nabokovian? I would argue that since Nabokov speculated that Jekyll and Hyde exhibits a possible "Victorian" suppressed homosexual element, that yes, it is in that sense Nabokovian.
Carolyn