Missouri was already a state during the era under discussion, having come in under the Missouri compromise in 1821. Generally what happened if there was a surplus (slaves died, and surpluses were rare), was that they were sold, but there was a lot of economic growth in what is today the South East and was then just the South thanks to the expansion of cotton production. There was no need to expand slavery to the west for economic reasons.
The slave debate is something that is generally framed wrong from both sides. While undoubtedly there were people on both sides who saw it as a moral matter ("slavery is evil"/"the Bible says slavery isn't evil), the simple truth is that the Democrats in the south had more money, and hence more political power and the Whigs/eventual GOP in the North didn't want them to have more power. The war was over the political advantages, in Congress, of this or that state being slave or free. The Southerners who pointed out this political distinction were denounced as slavers, even when they were pointing out that someone with a factory full of Irishmen being worked 7-12s under starvations wages had no legitimate moral position better than the slaveowners. Yes, there were idealistic extremists like Thaddeus Stevens; there a few idealistic moderates like Lincoln among the abolitionists, and there were extreme "pro-slave" Southerners such as John C. Calhoun, but there were also slave-moderates in favor of gradual manumission such as Robert E. Lee. So the war was about power politics, based around slavery, as much as it was about the actual issue of slavery. This is a nuance, but it is not a quibble. The war was fought over that argument of political power -- the "expanded market for slavery" was neither a proximate cause of Bleeding Kansas nor the war that followed.