(spoiler centric, sorry for another TDK topic...yadda yadda)

I've seen TDK about 10 times now (at work), and there are several points that I'd really like clarifying:

1. Jim Gordon's "death" - at Commissioner Lob's funeral, it looks as though he dies. Then, his colleagues tell his wife that he is dead, however, then he turns up driving the van supposedly transporting Harvey Dent to safety. Does anyone know that he isn't actually dead? Are the police joking when they tell his wife? As if they genuinely think he's dead, shouldn't they only have told her following medical consultation? Did no-one notice him just get up and run away?

2. Can Batman fly? At times he seems to be able to; at others, he's just falling with style. (Also, this isn't a problem, but he doesn't appear to have superpowers - is he just very strong? I've not seen Batman Begins).

3. Surely, with a rampant psychopath on the loose, there would be a heightened sense of security about the city - therefore, why is the Joker (or the insiders in the police force) able to poison Commissioner Lob, and rig the bottom of the ferries full of oil? Surely people would have noticed? I completely understand that comic book/superhero films don't always conform to reality, but the rest of the film is almost entirely feasible, and minutely detailed, so why these gaping plot oversights, or am I missing the point?

I do like the film in general though. Also, is there a graphic novel on which it's directly based, or did Chris Nolan come up with the majority of the storyline?