Possibilities surrounding nonverbal spells (esp. re: Snape & DD)

[cubfanbudwoman]

I've been thinking a lot about NONVERBAL spells lately.

On pp. 178-179 (US ed.), we attend the first DADA class of the year,
wherein the following occurs:

"
you are, I believe, complete novices in the use of nonverbal
spells. What is the advantage of a nonverbal spell?"

Hermione's hand shot into the air. 
 "Very well – Miss Granger?"

"Your adversary has no warning about what kind of magic you're about
to perform," said Hermione, "which gives you a split-second
advantage."

"
 correct in essentials. Yes, those who progress to using magic
without shouting incantations gain an element of surprise in their
spell-casting. Not all wizards can do this, of course; it is a
question of concentration and mind power which some" – his gaze
lingered maliciously upon Harry once more – "lack."


 "You will now divide," Snape went on, "into pairs. One partner
will attempt to jinx the other *without speaking.* The other will
attempt to repel the jinx *in equal silence.* Carry on."

On p. 217, we learn that:
"Nonverbal spells were now expected, not only in Defense Against the
Dark Arts, but in Charms and Transfiguration, too. Harry frequently
looked over at his classmates in the common room or at mealtimes to
see them purple in the face and straining as through they had
overdosed on U-No-Poo; but he knew that they were really struggling
to make spells work without saying incantations aloud."

On p. 238, Harry discovers the "Levicorpus (nvbl)" spell in the Half-
Blood Prince's potions textbook. Harry assumes that "nvbl" stands
for nonverbal and decides to try, without much expectation of
success, speaking the spell internally. It works, and Ron is
awakened by being jerked upside down "as though an invisible hook
had hoisted him up by the ankle."

All this emphasis on nonverbal spells has me thinking. Since the
textbook was Snape's
 since Snape is now one of the teachers
stressing nonverbal spells
 I'm assuming managing nonverbal spells
is something Snape is fairly good at himself.

Now, there are all kinds of questions for me about nonverbal spells
in general. Can *any* spell or incantation be successfully
performed nonverbally if the caster is talented enough? Are there
only certain spells which can be successfully "nonverbalized"?
Are there, as the Levicorpus notation might indicate, some spells
which are DESIGNED to be accomplished *only* as nonverbal spells?
OR is it simply that THE DARKER THE DEED, THE GREATER THE NEED to be
able to perform the spell nonverbally? (You know what I mean??)
I mean, Harry attempted Levicorpus the first time as a nonverbal
spell and succeeded. Would it have worked if he'd spoken the spell
aloud? I don't THINK we know the answer to this, do we?

All of this raises an even greater question for me, re: Snape on the
astronomy tower with DD. And here is that question: IF one is
accomplished at performing nonverbal spells, is it possible to
perform a nonverbal spell while *simultaneously* speaking the
incantation for another spell? IOW, can a highly talented wizard
manage to concentrate so hard upon a nonverbal incantation, that he
SUCCEEDS at it, even while speaking another spell which does NOT
come to fruition?

Could Snape have been performing a spell *other* than AK, while
speaking an "Avada Kedavra" which did not take because it
was "overpowered" by whatever nonverbal spell Snape was
concentrating on?

Yes, I know – the jet of light emanating from Snape's wand *was*
green, as one would expect with an AK. But
 is it possible that he
was thinking something else, actually performing something else? Is
there any other spell which produces a green jet of light?

Not saying I buy this at all. Actually, I'm in the camp which
believes Snape was following DD's orders in killing him – to
simultaneously save Draco from murder, release DD into the death
which was coming anyway from the potion, and to save Snape from
breaking his Unbreakable Vow. But many of us suspect some mutual
legillimency in that brief stare between DD & Snape. Could there
have been instruction to Snape about what to do in that moment? I
like the possibility of this.

Any thoughts??

Siriusly Snapey Susan

(forgive the cross-posting if you're also at HPfGU)