You say he's a conspiracy theorist and then jump to Russia gaining access to the emergency alert system? Seriously? Both these scenarios sound equally probably or improbable to me.
So the plausibility of Russia doing it would be based on whether they would bother or not.
It doesn't mean you stop looking for causes or eliminate human error though.
The viability of Russia/NK depends on how the EAS network is set up in Hawaii. If they have internet-facing servers, it's completely possible.
Or it was "just a prank, bro!"
That escalated quickly.