Well, since this is an AU fic, I'm going to take some latitude and make the Death Eaters a much more formidable group than everyone thinks, kind of like a secret cell of terrorists or Nazis.  You'll find out more about that later though in a forthcoming chapter, when Harry tells Sev about what he overheard while a prisoner.  And Dumbledore isn't infallible, also the Death Eaters knew about Harry and some of them, like Lucius also knew about the prophecy, so it makes sense for them to be waiting their chance to hurt him. 

As for the last part, yes I know that doesn't seem like the Snape the professor, but remember he wasn't always a teacher and sometimes he reverts back to his rather low class origins, especially under times of stress. 

hanks for your remarks though and hopefully I've answered your questions.