Now to my theory on Darl.
Okay, Darl knew his mother was dying and knew his family and how they would most likely react. I don't think it would have taken a genius or psychic to figure that out and that Darl is the most sophisticated thinker in the family. He continually thinks of what defines existence down to the linguistics level through his continual conjugation of Be. I point this out just to show that his is the thinker in the family.
Now, Addie's lover, Jewel's father, showed up at the Bundren's though we are not privy to any conversation between Anse and Whitfield it could be what finally cemented this though for Darl along said the way Jewel is treated differently by Addie.
Now the reason Darl knows that Dewey is pregnant, in my humble opinion, is because he went Quentin on her--it's his child. All the Lafe stuff is a cover-up. There is one single reference to incest in the book, it's near the end and I think Darl mentions it. That's how he knows that. What Vardaman saw and isn't supposed to talk about isn't that Darl started the fire. I believe he overheard that Dewey Dell was pregnant.
I could be wrong but there's not enough information. It's my explanation that takes the paranormal out of the story but that's what makes the story fun.