1-- Just some of the literature professors over-analyzed (hard to believe, I know-- but that is the writer's intent). If he were speaking of all of them, there would be a comma before 'who'.
2-- forward-referring 'the' before 'literature professors' is optional. Omitting it does seem to me to refer less specifically to the professors he is speaking of, which is diplomatic of him. No article can go between 'literature' and 'professors' here-- it creates a meaningless clause. The comma's effect I have already mentioned: it changes the restrictive (limiting) clause into a non-restrictive one.