Well this made me laugh.
What on Earth makes you think the US is any less interventionist that it's been in the past? This administration has increased both troop numbers in Afghanistan, higher military spending, and pushed for tighter sanctions against Iran.
Foreign policy wise Bush's first term was significantly more aggressive than his second. Obama's foreign policy stance hasn't changed a great deal from Bush's 2nd term. The same policies have continued relatively unhindered. Obama's justice department even pushed for an end to the right of Habeas Corpus granted to Guantanamo Bay prisoners.
Would they initiate a first strike against North Korean arms? No, but Bush wouldn't have either. They have to maintain their image in the world, however tarnished. The world still holds a lot against the US for invading the middle east. There would have to be a clear call to arms on North Korea's part before the US did anything, under either president. Besides, if the US attacked, NK would do everything in their power to wipe SK off the map.
It's politically better to let them make the first move, because then then North Korea can't play the victim card.
If North Korea makes a viable batch of nuclear weapons with a high enough reliability and yield to wreck some serious havoc the overwhelming chance is that they'd either give them off to extremist parties, or send in some of their own agents to make it look like a terrorist act. This way they can claim non-involvement and prevent the destruction of their entire nation.
Even madmen want to protect their ass.
As for what the Us should do, if North Korea attacks the South I'd take nuclear weapons off the table. Once they've been used once it gives other nations an excuse. It's well known that China has nuclear weapons. Plus, the US spends as much as the rest of the world combined on their conventional forces, they could annihilate them without the threat of a nuclear chain reaction.
Just my $0.02, if you feel like responding respond to it all, not just one line.
Thanks

eace