It would not mean the same. "mot" means there is a risk the city will be destroyed. "för" makes it sound like the volcano makes the people in the city feel threatend, but not nessecarily for destruction of the city. Since the latter is a less likely scenario, it is not common to use "för", I think it is like "to" in English.
Hot mot makes me think of a semi recent expression in the U.S., "hot mess". It is generally used to describe things ranging from a very bad hair day to the current presidential administration. Even the latter is a bit short of something like Vesuvius having a major eruption again. (I resisted any bad puns about hot. Varsågod. ;-)