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

[potioncat]

SSSusan wrote: 
> 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?

 – 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.

Potioncat:
I've only read the book once...Am I only one whose family would think 
I was crazy if they knew I was already re-reading it?...but I can't 
quite get the idea of another spell having any advantage. Dead is 
dead. 

I don't understand why JKR had DD falling off the tower and that 
seems to be the big hint about whether it was an AK. But if I were 
choosing for myself, I'd choose AK before a fall off a tower as my 
means to an end.

The only thing that might save Snape is if he knew DD had died and  
performed the curse a split second later. But still, why the fall off 
the tower?