Why does Dumbledore make Snape DADA teacher?

[Nate Hennessey]

No: HPFGUIDX 158389

Just a quick question, I'm not sure whether it has been discussed
before or not. If it has, I'm sorry. In the 6th book, why does 
Dumbledore seemingly relent and make Snape the DADA teacher?
I've come up with a few reasons. I would like to hear what anyone
else thinks.

a) the position Snape would vacate (potions) would be more
difficult to fill than DADA, so Dumbledore had to wait until
he could find someone willing/trained enough to fill it. 
(Another question - what had Slughorn been doing with his life
up until HBP, and why did he leave Hogwarts in the first place?)

b) Lucius Malfoy had finally been arrested. Perhaps Lucius would
have exerted some sort of pressure on Snape to modify the curricula?
(I can't remember, but was Lucius on the Board of Governors
 until the events in the Department of Mysteries, or did he lose
that position after the coercion tactics in CoS?)

c) Dumbledore had some reason to finally trust Snape with the
position/reward him with it.
(healing his hand after the ring horcrux? Warning the
Order about what Harry was doing at the Department of Mysteries?)

d) Dumbledore was trying to sweeten the deal with Snape over
the spying business (or, if you go with this theory, payment for 
'fufilling Draco Malfoy's task by killing me' by giving Snape what
he apparently always wanted.)

e) Dumbledore knew that Snape would have to leave at the end of
the year anyway, so there were no worries about the curse.

f) The way I see it, Dumbledore and the Board of Governors
grappled together over who they hired or didn't hire. 
In this case, members of the Board of Governors may 
have finally stopped blocking Dumbledore's attempts to 
hire Snape for the position.

There's no evidence for this, but think about it logically. 
Would they really want a known former death eater teaching their
kids DADA? If Lucius Malfoy was trying to distance himself from
association with Death Eaters (as he seems to do when he tells
Draco not to badmouth Harry so much in CoS), he wouldn't want
to promote a former one to a teaching position.

Though one also wonders if this were the case, why the Board
would promote Snape after Voldemort's return was verified. 
However, considering the spectacular mistake of hiring the
Ministry appointed teacher, Umbridge, they may have felt more
lenient.

Another question, did Snape actually want the DADA job? I know
some characters said he did, but did Snape ever say so? 
Snape did treat the other DADA teachers with disdain, but he 
seemingly had reason to (and there is evidence he treated most
people in that manner), rather than merely because he wanted
their jobs.

Quirell, he suspected, was trying to kill Harry and steal the
stone. (Why didn't he tell Dumbledore? Did Dumbledore just
not believe him, or was Snape out to prove it himself?)
Lockhart was an idiot.
Lupin was part of a group of kids who bullied him, and nearly
ate him when he was a teenager.
Moody, or Barty Crouch Jnr, was playing a former auror and also
suspected Snape of being a traitor to the Death Eaters. He
probably treated Snape with suspicion.
Umbridge was a megalomaniac.

Any thoughts, Nate who thinks too much and has a compulsion to
list her ideas.