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.
Yes, the broomsticks were explained--it was the reference that Scrimgeour made to the "someone" that really stuck with me. WVMM is right, though--HP did stun one of the Death Eaters when he was released from the body-bind--the DE landed on the stairs, and HP jumped over him. I think that is the DE that Scrimgeour is referring to (pardon my syntax!), and any Order member present could have questioned him afterward. I still think that's odd, though. There is no mention at all of any of the DEs being left behind, with the exception of the one who was accidentally killed. Wouldn't someone have mentioned it in passing, at least, later on, when they were all gathered at Bill's bedside? I think that it would be pretty big news, having one of the DEs. No mention is made.