WARNING: THERE BE SPOILERS BELOW:
These chapters further my beef with Dumbledore’s occlumency plan. By the end of the 7th book, we really come to view Dumbledore as a true chess master. He’s thinking years ahead and anticipating what many pieces on the board will do in response to the moves of others. Indeed, he is even willing to sacrifice pieces if it gets him closer to victory.
With that in mind, it seems like an un-Dumbledorian blunder to think that Snape’s lessons would actually help Harry prevent Voldemort from invading his mind. How could such a wise chess master miss such an obvious flaw in the plan?? That’s what bothers me about it.
Or . . . duh-Duh-DUH . . . what if it actually WAS Dumbledore’s plan for Voldemort eventually to enter Harry’s mind and lure him to the Ministry? What if it WAS Dumbledore’s plan that members of the Order be put at mortal risk? What if he saw all of that as an acceptable loss in order (pun intended) to bring Voldemort out into the open? And he needed Voldemort out in the open to motivate Slughorn to [with lots of coaxing and a little luck from Harry] divulge his memories to get the full horcrux picture.
This explanation is a bit of a stretch but it’s the only way I can reconcile such a consistent chess master making such a seeming error in playing chess: it’s not an error; it’s a feint.