"He always hinted that he had an ironclad reason for trusting Snape," muttered Professor McGonagall...
Maybe Snape and DD did make an Unbreakable Vow, which again leads to the question: Who sealed the Vow? Or was veritaserum used to prove Snape's loyalty? That wouldn't actually be ironclad. Over the years, Snape's loyalty could have shifted, yet again. I'm leaning toward a Vow on this point. As for Rowling's assertion that "Eagle-eyed readers will find hints in the final pages," I found two late-in-the-book passages I thought were interesting:
"Somebody Stupefied a Death Eater on top of the tower after Dumbledore died. There were also two broomsticks up there. The Ministry can add two and two, Harry."--Rufus Scrimgeour
He's right about the two broomsticks. I do not trust vague words like "somebody," especially when used by someone who has an unknown agenda, as I think Scrimgeour has. And from a page or two earlier than that exchange:
It was important, Dumbledore said, to fight, and fight again, and keep fighting, for only then could evil be kept at bay, though never quite eradicated...
I hope this is a general statement and not an omen. I have enjoyed the series but when it ends, I want it to end. No killing off VM without having had destroyed ALL of the horcruxes.
Harry supposedly stupefied the DE through the door, I thought. However, didn't Harry's DADA class work on casting a silent jinx? Perhaps Snape was silently doing that. Someone gave the thought that Lupin may have been the witness. I do think that Snape and Dumbledore had an unbreakable vow, that in order to stay close to the Dark Lord, Snape would do whatever it takes for the good of the order. I think DD knows that Voldemort would always want to face off with Harry, and therefore Harry would never be sacrificed for the Order....
The cup is the Grail...
It could be that DD simply trusted Snape because of his expressed love for Lily. Weren't the two broomsticks the ones DD and Harry took to the tower? As for the last quote, it's Churchillian. She'll kill Voldy, but probably at a price.