Russia has already demonstrated the ability to pierce Ukraine's air defense umbrella not even a kilometer from where Zelensky was. There's no need to do the dirty work of an assassination when Zelensky willingly goes to the front line every few weeks.
Russia has more than enough munitions for local air superiority, even if they can't maintain that across the front line.
The UN panel to monitor sanctions on North Korea ended in April. Australia has no mandate to monitor North Korea at this time.
The Yellow Sea is also not a body of water for which "innocent passage" is typically needed. It's a dead end body of water.
Moreover, if Australia was on UN business, there's an expectation that they should report their business to Chinese authorities to avoid this exact type of incident.