General Theorem: This series of chapters ought to be named "Tom Riddle and the Illusion of VIctory".

Voldemort has a nigh-absolute escape hatch. He can escape nearly any defeat, any trap, simply by dying. Possibly it's even worse than that, and he can abandon bodies at will.

He also has a strong tendency to discount the intelligence of anyone who is not him.

The order of the pheonix was operating under the theory that he was a body-jumper from the word go.

The traps laid, the strategems in place are predicated on the central principle of allowing Voldemort to continue to think he is winning until it is much to late, and his defeat has become truely inescapable.

And I am pretty sure he's walked into several of these snares already - In cronological order: Things that were likely traps not yet triggered. The DADA job. The corridor - in particular, standing around in Snape's chamber for a full hour. The trip through the mirror, donning the cloak. Picking up the stone. Heck, Hermione's corpse. (Harry should not have succeeded in sneaking that past Dumbles. So maybe he did not?) I'm probably missing several...