One possibility you may not have considered is the North Korea may actually supply Iran with ballistic missiles with fully operational nuclear warheads. Of course China would help disguise such international shipment from North Korea to Iran, and possibly supply the shipping cover to do so.Noth Korea would have plausible deniability saying that the Iranians built their own.
Things are about to get hot in the next few months or so. Our ears have to be very attentive to the role of North Korea and China support of Iran.
Iran has better missiles, so i could believe NK provided the plans or a potential actual warhead. I think Iran has plans based in large part on NK research and work (Iran paid a lot for).