Okay, well ponder this...

Dumbledore got Snape to teach Hary Occlumency, and never gave a reason. Maybe the reason was that Snape is a VERY well precrised Occlumens, and he is better than Voldermort, but Voldermort doesn't know that. It could very well happen.

If this is true, Dumbledore may trust Snape because - having this thing means he can bat for Dumbledore, as well as be an inside spy on Voldermort.

Also, that thing about him protectsing Harry is not a bad idea, and it could easily be what happens later in the 7th book.

This may mean, that in the 7th book, he tells that he may have had to kill Dumbledore to protect Harry, but this is a lot more farfetched idea than him protcting Harry.

But then again, why does Ron keep saying that he suspects Snape is dodgy? JKR would NOT have put that there for no reason. Everything in her books have a reason for being there.

Well that's me!