"As I Lay Dying"- clues into Darl's instability

In the last few chapters of the book, we learn that Darl set Gillespie barn on fire and that he is not well psychologically. The only possible reason for why Darl did what he did is in Cash's second to last chapter: that Darl set fire to the barn to burn Jewel's horse (take away its trade value). Darl's actions that were confusing in earlier parts of the book (for example, laughing at Addie's coffin) are now closely replicated (crying after the barn burned and laughing after he has been exposed) and can signal his instability growing. Darl is also described as a little odd earlier in the book, meaning that he could have been mildly unstable or even just different, in general, not just psychologically. His elevated vocabularly suggests an education that none of the other characters recieved, which is puzzling because Cash would have been educated first, thus probably gone father as the eldest son of a southern family in the early 20th century. It is interesting that Jefferson is the place they burry Addie, the home of the new Mrs. Bundren, the largest town closest to the Bundren's home, and the name of the prison/insane asylum (as they would have called it) that Darl is sent to because using only one name makes it seem that they are all the same thing: death, containment, and renewall. By his last chapter, Darl sounds incredibly like Vardaman. Readers can assume that he is in his cell in the Jefferson asylum/prison for insanity (that probably isn't what he had, but they really only had one mental health diagnosis in those days -- though mental illness diagnosis was slowly getting better). 

Perhaps, the philosophy of The Things They Carried had too much of an influence on me and this di super out-there, but Darl's omniscience, his intersection of his language into other's chapter (particularly Dewey Dell), and his comment to Jewel when he asks about who his father is ("your mother is a horse..") that matches exactly what Vardaman thinks/says, and his complete change in diction and syntax to mimik Vardaman's makes me wonder if this entire story was real or if only the basic facts and some of the people were real, or if all of this is what Darl expierenced and is recounting in his mind as he sits in Jefferson. The characters, who are probably real, could be talking as he views them and they change their diction, etc. when Darl's mind goes back to the way that he thinks, not the way the characters think; or the characters could all be real to an extent, but Vardaman's story isn't his but Darl's "insane" one because their syntax and diction are so similar.