i don't think quirrel was a death eater because Voldemort in his statement during G0F says that suddenly a young wizard stumbled onto him, He doesn't make mention of him being his servant and searching for him as he does with Wormtail. So I think when he says about the two one to coward to come back and one who left them forever, he is referring to Karkoraff and Snape. That is why those two were talking during the book.
I don't have my books here w/ me!!!!!! But, we didn't know about the order in Book 4, so probably not.
Or are you talking about in Book 5 Ada...I don't think those in the Order aim to kill...I think they would stupefy or something and let Azkaban take it from there. I think the only one who died in Azkaban was Barty Crouch Jr.'s mom (for those who don't remember they switched places via Polyjuice Potion and Crouch's House Elf was to watch Jr.) The others just go crazy or something while there (except Sirius b/c he knew he was innocent), and they only die if they receive a kiss from a Dementor...I don't remember, but did Crouch Jr. get the kiss? And what happened to Crouch Sr.? Now I have a question...what's up w/ Azkaban?!?! First Sirius, and then a whole SLEW of other Death Eaters escaped in book 5...were they apprehended in the end? Have the Death Eaters already gone to Voldemort? Dumbledore must hate being right about EVERYTHING!!!
Another question...will Voldemort be the bad guy in this book? Yeah yeah yeah, always the bad guy. But, in PoA Voldemort wasn't, it was Wormtail. And also, will Voldemort be stopped at the end, like he was in SS/PS and CoS...b/c he wasn't in GoF or OotP. If he goes free at the end of HBP he'll have gone free for THREE YEARS!!! The Order better be on top of this in the beginning of the year! What do you think will happen to make Harry leave the Dursley's? We've already had Dementors in the neighborhood...do you think Voldemort himself would dare to enter Harry's 'hood?