I won't disparage your point of view on this, and I'm quite certain as much as I love Dylan, if you took an entire course on the authorship of his material, you're probably more well-prepared for this coming in than I would be, but my two cents on some of what you're saying:
Woody Guthrie also paved the way for a lot of what Dylan did and was also certainly a prolific songwriter, but I don't picture Guthrie as being the literary or cultural force Dylan was (assuming Dylan is indeed the author, which I know for you is a stretch - more on that in a second). The way I see the big picture of modern music is that Guthrie (and folk music in general, but Guthrie in particular) paved the way for Dylan, and Dylan paved the way for the Beatles. I think the reason Dylan gets more credit is because, as much as Guthrie was important, he didn't capture the American imagination or drive the cultural scene the same way Dylan did in the 60s and 70s. And the reason I think Dylan was more important than the Beatles was because he paved the way for them AND made an impact himself on the music and cultural scene. In a nutshell, my feeling is no Dylan, no Beatles. Or at least that the Beatles would've been limited to being an above average pop rock group and not the cultural force they were.
As for the authorship, I know that questions of what he wrote and what he didn't write have followed him for years, even to one of his more current albums, Modern Times. I think in all the cases there's a fine line between borrowing/adapting and stealing. I think he recognizes the folk tradition of taking something from the past, adapting it, and making it something new. I don't think he ever tried to hide the fact, for example, that "Girl from the North Country" is a riff on "Scarborough Fair." He uses the same melody in another song ("Boots of Spanish Leather," maybe? I can't remember). But even if you write off some of the more questionable songs, I think you still have songs like Desolation Row, Gotta Serve Somebody, When the Ship Comes in, Masters of War, Not Dark Yet, Cold Irons Bound, Make You Feel My Love, Lay Lady Lay, With God on Our Side, Only a Pawn in Their Game, Like a Rolling Stone, Ballad of a Thin Man, Tombstone Blues, etc. etc. etc., that still I think attest to his literary, artistic, and (dare I say it) musical genius.
To be honest, I don't know enough about the Nobel Prize for Literature to say authoritatively if he deserves one. If other poets have received them, I think it's a fitting selection. If not only songwriters but also poets have been excluded in the past, then yeah - I could grudgingly admit that it's a stretch.
BTW, let me know if I'm reading too much into this, but is your point more or less that a musical genius can't come from the ranks of pop, rock, folk? Because if that's what you're saying - well, I'd disagree, but I'd totally get where you're coming from. Hendrix was amazing, for example, but was he Mozart? I don't want to put words in your mouth, but it seems like that's the type of thing you're going for.