Southern states outlawed the importation of slaves because their slave populations had become large enough that they could breed enough for their needs without having to rely on Northern slave traders. The plantation-heavy states were still importing slaves from Kentucky, which had an overabundance.

As to free states vs. slave states, the point is that the FSL and the Dred Scott decision meant that it *didn't matter* whether a state designated itself free or not. Slave holders wanted the rights to 1) pursue their escapees into other states, whether that violated said state's law or not, 2) take their own slaves with them into 'free' states, ditto, 3) sell excess slaves into any state they pleased, ditto. By increasing the population of non-voting-but-still-counted slaves into other territories, they increased their political power. Which they needed to do, because the Northern states were having a large influx of immigrants, mostly Irish.

The assertion that slavery wouldn't have worked in such agricultural regions as California and Oregon I find extremely puzzling.

If you want to argue that Southerners valued their individual rights over states' rights, that I would agree with. But trumpeting 'states rights' at the same time you're trampling over other states' rights is rather. . .obnoxious. And disingenuous.