I think you are correct in your theory about Dumble's death. I actually belive that he was pleading with Snape to kill him not to save himself. I think Snape really is good (I belive in Dumble) and when he made the unbreakable vow that Dumble agreed to Snape killing him (did anyone else noticed that Snape saved Harry during his flight from Hogwarts?) . He knows this is the only way for Harry to continue and kill Voldy. This also just stregthens Snape cover for the Order. Just as Dumble ordered Harry to keep making him drink the liquid, I think Dumble also ordered Snape to kill him in front of the Death Eaters. He sacrificed himself for the Order and for Harry and for the good of all wizards.

But this is just my theory...