The main issue that I have with Trump's North Korea policy is that he's threatening a pre-emptive strike which can never be justified unless you have evidence of an imminent attack. Threatening to attack North Korea if they keep insulting and threatening the US is completely insane. I think that the UN sanctions resolution is probably the right path to go as long as you make it perfectly clear that if they don't follow through on those sanctions then China itself will be sanctioned.Threts against North Korea is never going to work because the regime views the nukes and the strategic reach as their only way to survive and not go the way of Saddam and Gaddafi.
The issue with Seoul being in range of artillery has been around since the Korean armistice, so there hasn't been any workable solution for roughly that long, unless you find millions of dead South Koreans an acceptable price to pay, which going by your previous posts I assume that you don't.
To actually do it yes, but they haven't and there's roughly 0% chance of it ever happening because they know it'd be suicidal, North Korea wants nukes for self protection and deterrence just like every other nuclear state before it.
Yes that'd be more productive than threatening them on national television because those threats only give North Korea more propaganda material as well as entrenching Chinese support for the regime because China very much don't want a US ally directly on their border.
Obama actually ensured that Iran won't get nukes because under the deal they have to get rid of their weapons grade Uranium and Plutonium and dismantle their centrifuges. So Obama's deal basically reset Iran's nuclear ambitions to zero as long as they comply with that initial phase.
I don't think that either North Korea or Iran having access to nukes is any worse than the US or Israel having nukes. The US to this day remains the only nation to ever use nukes against another nation, and the US has done it twice! And against civilian population centres, in contravention of international law no less.
The problem is that there's no workable solution unless you accept the sacrifice of millions of South Koreans civilians and hundreds of thousands of US troops & South Korean military casualties, the current US DoD estimate is 200k-300k military casualties within the first 90 days of conflict. And that is assuming China stays out of any such conflict, which it says that it won't if the US initiates a first strike.