Why could southern politicians be considered over-optimistic in accepting this deal? what might they have been hoping for in the future if they supported the spread of slavery?

They ensured a huge free-state like California while giving the regions a chance to decide in favor of themselves.  

They trusted the regions would cast a ballot to permit subjugation.  

They trusted that if the regions cast a ballot to permit subjugation they would hold an enormous populace and various states the regions made up more land than California.

the constitution of the united states gave congress the absolute authority over immigration laws

b toward the volcano.landward. the landward side of something is the side nearest to the land or facing the land, rather than the sea.