Those who have them are effectively safe from everyone.
No, unless they show they're perfectly willing and able to use them. A weak government will simply have them taken away either through sanctions or through conventional military strikes. There must be no doubt whatsoever that those weapons will be used, quickly and without targeting limitations, in the event of aggression and that the damage inflicted to the aggressor - and its homeland, industrial facilities and civilian populace - will be crippling and unrecoverable for at least a generation or two. It's the same as confronting a thug in a dark alley: showing off your holstered gun and whispering in a trembling voice "Ah... Ah... I'll... Shoot you..." won't work. Drawing, aiming at the head or chest and beginning the trigger pull will: either the thug breaks, or the trigger breaks.