I am pretty confident that SK, Japanese, and US layered defense of THADD, AEGIS, GBMD, and PATRIOT could effectively knock down a NK missile or warhead, wherever it is aimed.  However,  ripple launch of missiles in every direction may not be as effective.  Of course, this would result in massive retaliation against NK.  There may even be intelligence warning of this (despite mobile NK missiles) and that would require a pre-emptive strike (assuming that doesn't lead to WWII with China and Russia).