in Diablo 2 it refers to the "battle of Tristram", implying that was just more than one person involved. In the rogue's camp, they also talk about numerous rogues going to Tristram.
Having taken the time to sit down and watch the 2011 Blizzcon Lore Panel* (and having never read the Book of Cain) I can say confidently that I like OVERALL the story arc and origin story for the world of Sanctuary that they're going with. You are right, and I was mistaken; The Rogue is Blood Raven and the Summoner is the original Sorcerer (I was 11 when Diablo 2 came out, so some of the finer details I did not catch, I will admit). Being now older and wiser, with a better eye for storytelling, Diablo 3 still falls short.
Tyrael was the symbol of justice, and later became the symbol of wisdom. Justice and wisdom would still exist in the world without a symbolic angel sitting there in Heaven. Similarly, the defeat of Asmodan doesn't mean that everyone will stop sinning.
Unfortunately, this is just not the case. It's pretty well established in the lore that all of these beings are, for all intents and purposes, immortal. When they are 'slain', they simply vanish into the Abyss to slowly reform and regain their power. When Tyrael CHOOSES to become mortal (though I'm still not sure how, as Metzen says in the lore panel, Angels ARE their wings; their wings are not only emblematic of their nature, they are the ethereal housing of their essence and intelligence) he is no longer Justice incarnate, and as is explained in the game, this has terrible ramifications in Sanctuary. With his fall, it is soundly stated that the laws which bind angels and demons no longer affect him (the Crystal Arch and all that...)
Azmodan, Diablo, Baal, all of them still emit their -essence- even when they are killed in battle, in much the same way that the base materials of a star still exists when it goes supernova, eventually regaining its physical shape. This explains why the events of Diablo 3 happen twenty years after Tyrael is vaporized, and not immediately after he destroys the Worldstone.
This is where they start to mess up the plot. Because the Worldstone no longer exists on the mortal plane, the Lords of the Burning Hells do not need a human body to take control over like Diablo needed Albrecht (once again stated in the lore panel). This being soundly the case, WHY does Diablo 3 need this convoluted plot for the Lord of Terror to take over Leah? It's worse than unnecessary, it's counterproductive. Without that critical point in the plot the Warrior does not need to be Leoric's older son, Adria does not -need- to have been swayed by the Dark Wanderer, and Leah need never have been born, or even introduced. As somebody to take over as lore-keeper for her uncle Deckard, this would have been fine. But she is killed off before we even get a chance to invest in her emotionally, making her character, sadly, completely benign and pointless.
The events up until now are not nearly as moving as they could be for the amazing universe Blizzard North created. BUT! It sounds like they still have something up their sleeve, so with one or two expansions, it is completely possible to turn this tragically underwhelming plot around. It can be done!
*To be found here: http://www.youtube.com/watch?v=vutV77rKXXE