Did Snape have a choice or not? Levels and contradictions in JKR's writing

[pippin_999]

No: HPFGUIDX 138637

> Jen: He has emotional control during the scene, but look where he 
ended up! On the floor, kneeling in front of Narcissa and sealing 
his doom. What in the world would make him do that? He doesn't need 
to prove himself to either of these women if Voldemort trusts him
as  much as Snape says he does. He can look out for Draco without an
UV.  Something motivated him to make this huge error and I'm just 
searching for what that could possibly be. 

Pippin:
Dumbledore's orders?

I'm interested that I'm not the only poster who thinks Narcissa's 
lines sound rehearsed and calculated. 

The conditional language  "And should it prove necessary...if it
seems Draco will fail" sounds more like Dumbledore trying to be 
clever than a desperate woman pleading for her son. As Dumbledore 
pointed out to Draco, it would not seem necessary to kill someone who 
is already dead. (This clause  also allows Snape to survive if 
Dumbledore fakes his death or dies of other causes.)


Also, Narcissa's tears flow with what seem to be unnatural volume, 
soaking Snape's chest and all. They  start after she's
had her second glass of wine (which Snape poured) and end with 
suspicous suddenness after her third. Did one contain a  tearfulness 
potion and the other its antidote? I don't believe we've heard of
tearfulness potions specifically, but there certainly are ones for 
other  bodily, er, secretions. 

In that case, the whole scene is a drama staged by Snape and 
Narcissa for Bella's benefit, as a way of getting the Dark Lord's
blessing, so to speak, for Snape to watch over Draco. 

Since Voldemort is incapable of trusting anyone, and he doesn't 
need Snape's help to carry out the plan, he would allow 
Snape to watch over Draco only if  if he had no choice. 

That Snape tells Draco he made a UV suggests that he isn't
concerned to have Voldemort learn that he made it. That in
turn suggests that he expected Bella to try to earn her way
back into Voldemort's favor by telling him about the vow --
she'd sacrifice her own son for Voldemort, she says, so
why not her sister? And she obviously has no love for 
Snape.

But Narcissa is safe for the while, because Voldemort needs her
as leverage over Draco, and Snape cannot thwart the plan
without, Voldemort thinks, being forced to carry it out himself.
Voldemort need do nothing but await the repair of the
cabinet. That is the real kicker, the part of the plan that
neither Snape nor Dumbledore guessed, that Voldemort actually
had a reasonable expectation of getting his DE's into the castle.
Even Harry's suspicions did not extend that far.

Pippin