This is a very interesting and quite well thought, but I'm with the others here in asking "If it was the Draught of Living Death why was he awake the whole time?"

And yes, Snape did say the havad kadavara out loud, but that doesn't mean that is the spell he cast. Silent spells have been heavily emphasized in this book. He very well could have cast another spell silently and said "havada kadavara." Especially since we already know you can't merely say the words to the havada kadavara, you have to really mean it, really hate someone. So Snape just saying it if he was really casting anther spell woudn't have affected anything.