@#1: If you've been a fan of the series for any amount of time, you know that it's kind of a thing with Tyrael for whatever reason. He only destroyed the Worldstone as a last resort. I think his caution plays into his role as the Aspect of Wisdom now. Destroying the stone that houses every prime evil could carry serious repercussions--apparently, destroying the original soulstones on the Hellforge did not destroy the primes like was previously thought. At least if they're in a rock, you can move them around and know where they are. Until they get stolen, that is :3 But at least they're not acting of their own accord, exactly.
@#2: The obvious answer (which you know and we all agree is meh) is because #storyreasons. If you could be one-shotted, there would be no story. It would be over. I haven't played through RoS yet, but I'm guessing it's probably because you're a nephalem. That's the general catch-all explanation.
@#3: Yeah, that's kind of silly.
@#4: Can't speak for this since I didn't play the expansion yet. I know inThe Black Road, Bramwell is regarded as a crowning achievement in the west for its dense population and thriving economy once Kabraxis' church becomes popular. If it's still a haven of men, it could be strategically useful to destroy it--destroying hope, a center of power, chaos, etc. But I'm guessing none of that was said in-game. The old books aren't viewed as canon by Blizzard, anymore.
@#6: More lols. He also has a poor voice actor.
@#7: Well, in his defense, apparently the vaults of the High Heavens weren't even safe. So safe-guarding it might be a moot point.
@#8: I would assume that although he remained a mortal at the end of D3, he re-assumed some level of portal or utility when he became the Aspect of Wisdom. I inferred that he resumed a place on the Angiris Council, although arguably that was never specified.