With all due respect (and in your case I can assure you that is a lot!), I think that this is a rather fanciful interpretation! Pigs may indeed be more "moral" than humans, but I think that the nature of animals is entirely foreign and "other" to human experience, and cannot be compared so directly..
Alternatively, I believe this episode can be reconciled/interpreted in a similar straightforward manner. For instance, although God's permitting pigs to be possessed is a dreadful matter, it is surely no more dreadful than God's allowance of humans to be possessed in the first place. (This would especially be the case, if it is true, as you suggest, that humans are more defenseless against the forces of evil than are swine!) While you indicate that humans revel in sin, I take exception to that, and doubt that very much. Sin is not a glorious condition, and I truly believe each and every human wishes to escape our ignorance into the beautiful moral order of Christ.
Although sin and evil are no doubt plentiful among the human race, it is not as though the animal kingdom is not without it's share of fallenness. Indeed, many animals participate in behaviours which are downright horrendous, including unrepentant carnivorism -- sometimes even cannibalism (of their own young!).
For this reason, I think we need only maintain not that the pigs were "holy" and "godly," but that the same God who allows problems among humans (including those termed "demonic") may allow similar maladies to occasionally affect and infect the animal kingdom.