It's certainly true that immediately after that decision the dominos began to fall - the Panic of 1857, Bloody Kansas, the raid on Harper's Ferry, the Texas Troubles. The Buchanan Presidency must have been a time of disorientingly bad news.
This seems logical, as the essence of slavery seems (to me) to have been the denial of legal recourse to the enslaved, and the Dred Scott decision made that permanent and unfixable by any legal means. From that point on no problem between a slave and a free person could be legally addressed, which seems to me to lead directly to a society boiling over without any vent. Naturally, it's going to burst out everywhere.
However, it's rare to read anything from the time that mentions Dred Scott! For example, I'm reading the collection of period publications and writings called The Civil War: The First Year Told By Those Who Lived It (excellent book, BTW), and the only person who mentions the decision.....is James Buchanan!
(Who you would think would be the last person to bring it up, but he never saw it as him improperly influencing a Supreme Court justice. He saw it as the application of his Solomon-like wisdom to solve a problem forever and he could never see why it didn't settle the question.)
Were the people of the day right to dismiss Dred Scott as irrelevant, or was the true importance of the decision obscured at the time?
Your basic assumption-- that the writing of the period didn't mention Dred Scott-- is plain wrong. You should be focusing your searches on the period immediately following the decision, roughly 1857-1859.
If you really want to get into it, check out the records from the Illinois senate race of 1858 for a good example. There, Abraham Lincoln ran against Stephen Douglas on a platform that tried to whip up fear that the next Dred Scott decision would extend slavery to the North. Lincoln's famous "House Divided" speech is exactly the rhetoric that was common during the period. http://www.pbs.org/wgbh/aia/part4/4h2934t.html
You should also have a look at the Lincoln-Douglas debates, which the National Park Service conveniently has up on their website. http://www.nps.gov/liho/historyculture/debates.htm
The Civil War was not a slave uprising. It was not fought because slaves and slaveowners were at odds, it was fought because abolitionists and slaveowners were at odds.
I think it was pretty plain already at the time of the Dred Scott decision that slaves had no rights that a white man was bound to respect. As early as the 1830s there were massive rollbacks of slaves' rights to organize and educate themselves. Slave issues were hardly ever legally addressed. The Supreme Court merely codified what many intuitively knew already.
Also, the Supreme Court decision is somewhat arcane in its effects, while other sparks such as popular sovereignty, popular books, and attempted uprisings naturally captured more of the public's imagination and provided more fuel to the fire. I certainly wouldn't call Dred Scott irrelevant, but as a proximal cause for war it does pale in comparison to several other events.
i think your really underestimating the role it played and how it fit into longstanding grievances about Slave Power (and the attack on the venerable missouri compromise). I would recommend reading something like Potter's The Impending Crisis 1848-1860, or another high quality book about the era which places it in the right context.
From that point on no problem between a slave and a free person could be legally addressed, which seems to me to lead directly to a society boiling over without any vent.
one key problem is opposition to slavery had a large component about the problems slavery had on free whites in terms of economic losses and rights curtailed and in that sort of situation dred scott isn't the bright red line
your right on Buchanan though.