Here's the thing about stuff like this: why NOT create a story with this kind of secret behind it, especially the thing about Marcellus Wallace's soul? I think it's been tried, specifically by Verhoeven in Total Recall...
Total Recall 1990
ShowIs Quaid really a deep cover secret agent or is the second half of the movie him experiencing the schizoid embolism the doctor warned him about?
...and by Spielberg in Minority Report...
Minority Report
ShowDoes Anderton really escape prison and trap Burgess with help from his ex-wife or is the third act him experiencing the rapture of the prison containment system and all his dreams are coming true, as described by Gideon, the crippled jailer?
...and Ang Lee in Life of Pi...
Life of Pi
ShowDid Pi spend months in a lifeboat with wild animals or was his alternate story about being trapped with violent crewmen and resorting to cannibalism the real truth?
I think all three of these are a bit cheesy, really. On the other hand, the secret story underlying Mulholland Drive really enhances that story, altho it's so complicated I can't recall the details right now.
Warning: I'm probably rewriting this post as you read it.Zarban's House of Commentaries