Yes, which of course makes no sense (to me, at any rate) since out of the Union, there would then be no obligation whatsoever for federal enforcement of the Fugitive Slave Law which would drive the cost of slavery even higher.
You're absolutely right. In fact, IIRC, there were northern abolitionists who advocated northern secession for basically that reasons (EDIT: IIRC huh! Paul already said that at the top of this very page). And also to distance themselves when divine retribution started raining down, of course.
So what I'm wondering is why was such a (seemingly) irrational
rationale given as the cause for separation--and how many Southerners
really believed separating from the Union was necessary to protect
slavery?
We're way outside my competence here, but I'd speculate that the slavery rhetoric was just to rouse the rabble, who didn't think past the Northern "provocation" to consider points like you mention above. The usual "our whole way of life is threatened" schtick. Much like today. The folks doing the rabble rousing were I think much more focused on the fact that northern policies produced a massive wealth transfer from south to north.
Neglect of the fugitive slave laws imposed a cost on southerners, but it wasn't nearly the biggest cost. The Georgia declaration of cause cites federal subsidies of fisherman ($500K in that day's dollars), protectionism of shipbuilders, federal funding of buoys, lighthouses, etc. ($2M in that day's dollars), subsidies of postage primarily benefitting manufacturing interests ($7M, according to the document), etc. Money quote: "... free trade, low duties, and economy in public
expenditures was the verdict of the American people. The South and
the Northwestern States sustained this policy..."
--Len