but as for my theory with snape, i dont think it was part of dumbledores plan to have him continue to work with the order. I think snape was going to become a lone ranger in the Death eaters, cause it would be way to dangerous for him to still be in contact with the order even when it was him who killed dumbledore, it would be too suspisous. so the plan would be that snape was working alone. he wouldnt have informed the order so as to keep the illusion. now that snaped killed dumbledore the order would be after him, if tey knew however that it was planned they wouldnt show the same passion. we'll see what happens in the last book.
to add to this i always got the impression that dumbledore trusted snape with his life. this offers more evidence that he would even go through with this plan.
but there are alot of points that lead evidence the Snape is a bastard. i personally think that snape is evil however there are so many hints to say otherwise that they cant be ingored, my gut is saying he is good, my mind is saying hes a jackass.