I'm thinking of this line in OotP when Bellatrix tells Harry he really needs to want to cause pain to use the Cruciatus curse to its full affect. I suppose that, with enough intent, that some other hexes, jinxes, or curses could kill besides AK. We only know that AK is the only curse designed to kill instantly, but I assume that it must also be said with the real intent to kill or else it probably wouldn't work, either.No, I think that Molly simply used a curse, or hex, or whatever, just so powerful, that it killed Bellatrix. Not to mention that Molly had every intention of killing Bellatrix, which probbaly had an effect on the spell as well.
As for Sirius, the final spell Bellatrix hits him with is not described. I think Harry does not believe he's dead because maybe Bellatrix did not hit him with AK. However, after he fell through the Veil, everyone else is pretty certain Sirius is dead.