Why did Dumbledore allow Draco to disarm him?

Question

Related: How was Harry supposed to defeat Voldemort in Dumbledore's original plan? by @Arthur

Related: What were Dumbledore's actual intentions for the Elder Wand? by @NominSim

This question is not -- is not -- asking what Dumbledore's plans for the Elder Wand are or who was supposed to get the Elder Wand (We already know Dumbledore originally wanted the wand to go to Snape; I don't at all doubt this was Dumbledore's intention.). I'm asking why, after such detailed and calculated planning, Dumbledore so abruptly (at least it seems so) allowed his intricate plan to be derailed by Draco Malfoy?

Harry hurried over to the door leading to the spiral staircase, but his hand had only just closed upon the iron ring of the door when he heard running footsteps on the other side. He looked round at Dumbledore, who gestured to him to retreat. Harry backed away, drawing his wand as he did so.

The door burst open and somebody erupted through it and shouted: ‘Expelliarmus!’

Harry’s body became instantly rigid and immobile, and he felt himself fall back against the Tower wall, propped like an unsteady statue, unable to move or speak. He could not understand how it had happened – Expelliarmus was not a Freezing Charm –

Then, by the light of the Mark, he saw Dumbledore’s wand flying in an arc over the edge of the ramparts and understood ... Dumbledore had wordlessly immobilised Harry, and the second he had taken to perform the spell had cost him the chance of defending himself.

_{Half-Blood Prince - pages 545-546 - Bloomsbury - chapter twenty-seven, The Lightning Struck Tower}

I have a bunch of ideas regarding why Dumbledore sacrificed The Grand Plan in order to ensure Harry's safety, but certainly I haven't thought of everything. So I am asking the site:

Why did Dumbledore allow Draco to disarm him?

_{I'm looking for an answer grounded in canon (Harry Potter books or J.K. Rowling interviews/quotes or Pottermore); subjective answers in the spirit of canon are also very welcome. I am not looking for an answer from the HP Wikia.}

Answer 1

1. I think the question may be based on the fact that you and Dumbledore see different priorities:

   why Dumbledore sacrificed The Grand Plan in order to ensure Harry's safety

   The Grand Plan was hinged NOT on the Elder Wand or its ownership. It was hinged on Harry finding and killing all Horcruxes, and then sacrificing himself by his own choice, only to be resurrected in 3 days (src: Nicene Creed) freed of the soul piece in himself (src: DH, King's Cross).

   Yes, having the Elder Wand being owned by Harry, in the end, helped out a lot in the last fight with Voldemort. BUT, that was NOT a necessary part of the Grand Plan - the Grand Plan could probably survive even Harry choosing not to get resurrected, as Dumbledore offered him to do in King's Cross - after all, Tom Riddle was now mortal and could be killed by someone else as well.

   So, at the end of HBP, Harry's safety was super paramount. Nothing else was (witness Dumbledore insisting on using his blood instead of Harry's in the Cave in HBP).

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2. Having said that, I think the answer to your question is actually even easier than my last point:

   Why did Dumbledore allow Draco to disarm him?

   Because he didn't really win anything by NOT allowing Draco to disarm him.

   He knew/thought that DEs were in the castle, because he saw the Dark Mark. So, he knew he was about to be killed anyway, given his extremely weakened condition after drinking the locket-protecting potion. Whether Draco disarmed him with the first Expelliarmus or not wouldn't have really change anything - and it was tactically better to be disarmed by Draco as opposed to a more dangerous Death Eater as far as the Elder Wand ownership is concerned.

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3. Another factor - though I have no direct canon support for this - is that Dumbledore didn't know/realize/thing-at-the-moment that (a) Draco would disarm him or, more likely, that (b) Draco disarming him - but not killing him - would cause the Elder Wand to switch allegiance to Draco. In other words, he may not have realized that mere disarming spell would count as a "defeat" for the Elder Wand, as opposed to a real duel.

Answer 2

What makes you (and the other answers) believe that Dumbledore chose to allow Draco to disarm him at all ?

At that point in the story, he's very diminished by the potion and everything that just happened, and his priority (for some reason) is to impede Harry, presumably to allow Snape to kill him. So I don't see any reason to believe Dumbledore wasn't just taken by surprise.

Also, "allowing" yourself to be defeated kinda ruins the purpose of a defeat. For example, in HP4, Harry strikes Ron with Stupefix to practice for the third task, but Ron allows it. That kind of circumstances where the target is willing doesn't seem like counting towards defeating someone.

Answer 3

I personally think that Dumbledore has already known that Draco and Snape are going to kill him, therefore he lets Draco disarm him, without anyone knowing and makes Snape kill him so that everyone including Voldemort thinks Snape is the master of the elder wand, meanwhile Dumbledore might know that Harry would sometime disarm Draco and own the Elder Wand. Thus, the Elder Wand doesn’t fall on Voldemort’s hand.