Misunderstood Snape

[Cindy]

No: HPFGUIDX 134917

After doing a bit of rereading, I think chapter 2 is a better 
indicator of how to gauge where Snape's loyalties lie.  The 
Unbreakable Curse--okay, I think the whole point of it being called 
the Unbreakable Vow is that once you make it, its done--if event A 
then event B happens. Unless I've skipped it, I don't find where the 
failure to keep the vow is an option.  Draco couldn't do it so 
therefore Snape MUST do it, whether he abhored or loved the idea.  I 
think I've read elsewhere on the list that Snape either had to keep 
the vow or die--I'd like someone to point me to where that's found 
because I can't find it in HBP.  Why would Bella's jaw drop if it 
wasn't so final? Three bonds of flame bonding them together. Draco 
says on page 323 that Snape will have to break it, but he says it in 
a very flip sort of way, as if the vow won't be needed because he 
won't fail.  

If you look at it in this context, then Snape may very well believed 
he could prevent Draco from setting the events into motion by 
keeping an eye on him.  

Another thing I have been wondering about:

pg 604 Am. Ed.

"You dare use my own spells against me Potter? It was I who invented 
them - I, the Half-Blood Prince! And you'd turn my inventions on me, 
like your filthy father, would you?"

How did James know Snape's spells to use them on him?