MIRVs are definitely on their to-do list, but I don't think they're there just yet, the Hwasong-14 is supposedly designed for a 'heavy payload', so MIRVing it is not out of the question, but I think by the time they've gone into MIRV technology they will probably have moved beyond the Hwasong-14 and gone into a cold launch solid fueled ICBM of some description.
What concerns me about Russia and missile interception is that Russia has so far misidentified most of the DPRKs launches, insisting that they haven't launched an ICBM yet. Now either they're doing this for political reasons, or their radar coverage of the DPRK isn't that good. Which makes one worry that if they miss any launch from the DPRK that the Alaskan GMD intercepts then all Russia is going to see is a load of ICBM like launches from Alaska that are going to land on Russia.
