The missiles Hizbollah are using are one step (maybe two steps) better than pop bottle rockets. In other words, no way are they capable of boosting a nuclear warhead. Nor is the technology in the M.E. (Iran) developed to the point that they could design and build a nuke warhead that could be fitted to a small - read that rudimentary - missile.
Without discounting the possibility that Iran does possess one or more nukes, the only possibilities I see for Iran or its allies to go nuke on Israel would be a very lucky suicide mission by a MIG that somehow managed to evade US intercept, or more likely by a nuke smuggled in by sea and detonated inland - again with a lot of luck involved.
The bluster of Iran having a missile capable of reaching Israel is probably just that, 80 percent bluster. The possibility of their having perfected a nuke warhead small enough to fit a midrange ballistic missile is, IMO, pure fantasy. They may be close to, or even have a nuke that would go Bang, but I would bet real bucks that they are a long ways away from a missile deliverable warhead.
Thoughts?
I think you are right on....a terrorist nuke in haifa harbor probably most likely...smuggled from Pakistan/Iran to Lebanon for assembly, and then pirated South a few miles and boom!