Again, I have no information into what's really going on, but it seems to me that the Iranians are less likely to order Hezbollah to use the WMDs in Lebanon if the Israelis are only attacking Hezbollah, and not Iran directly. Use of WMDs by Hezbollah would require Israel to directly strike Iran, and Iran may not want to actually suffer an Israeli attack at this point. They don't yet have their bomb.
Basically, cleaning up Hezbollah removes one of the weaker parties without escalating into an all-out regional conflict. As long as the conflict is "low intensity", WMDs will probably not be used.
The price of using them is too high and is not justified just to protect Hezbollah, though Iran would probably feel justified in using them to protect itself.
I would hope that Israel would take control of the WMDs buried in Lebanon prior to launching a strike on Iran.
"Normally", I would agree with you. But keeping the straits of Hormuz in the back of my mind and wondering just how much the disruption of world wide oil flow actually plays into the minds of some leaders, I question if that would happen.
I'm almost surprised that we haven't urged the Israelis to try and secure them now. Maybe we really don't know exactly where they are. Possibly a more likely scenario is that U.S. and Israeli troops would move to secure the WMD's where ever they are. and best guesses seem to indicate they are buried in syria and Lebanon.