Re read the scene with Mrs. Malfoy and her sister. Snape could easily have known nothing but been drawing information out of Mrs. Malfoy. Dumbledore tells Draco that he knew about the murder attempts from the beginning of the year. How could he have known? Snape told him. Snape could easily have killed DD only because of the promise he made Mrs. Malfoy. He was not part of that night's plan. He also saved HP's life. What he told the other deatheaters about LV wanting HP for himself could easily have been a lie told to save Harry's life. We don't yet know for sure whether Snape is evil. (Although the odds are that he is.)
That's exactly what I was thinking when I read that chapter. I think that Snape kept DD informed on everything. When Snape and Malfoy left that party, Snape was desperate to get Malfoy to include him (or at least make him aware of) his plans. I don't think it's because he feared Malfoy as a rival--I think he'd sworn to DD, just as he'd sworn to Narcissa, that he would protect Malfoy as well as HP at all costs, up to and including killing DD himself.