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.