Well, I think your point is very smart. And I agree with you in most of what you think but not completely. First, I do think Dumbledore is dead, i seriously doubt that he would fake his own death. He is no a person that would hide or anything like that. But I think he KNEW that he was going to die that night, and wanted Harry to witness it all. I also think that Snape was acting under Dumbledore orders. Maybe the spell had other effect or appereance because Dumbledore is tha most powerful wizard and his energy would be different from the common wizards.