Q&A 2, Second Answer

The basic form of my question is: If a piece of writing produces unpleasant impressions, but nonetheless stimulates the imagination and sense of the aesthetic, would Steker still consider it literature?

I think that (after a little consideration at least) he would.  He might refine his view to remove the parts which seem to state that reading literature must produce pleasant stimulation of the imagination, and replace them with something more neutral - he might say that works of literature should simply be able to stimulate the imagination to some extent in some way.  Thus, works with vivid but highly unpleasant imagery would still fall under the banner of literature.