"I believe the southerners of 1860, at least some of them, thought secession was legal and among the options"
It definitely was NOT just "some southerners" (sic) (capital S if you please)
Being that insistent must mean that you have some information on that claim. It be nice to share some of it. And, by the way, if you want southern and southerners capitalized, you might lead the way by capitalizing northern and northerners.
Characterizing northern sentiment as solidly against secession in principal is mendacious (whether intentional or no) A textbook in use at West Point (in the years R.E. Lee attended) reads thusly: "The secession of a state from the union depends on the will of the people of such state." He adds, "It depends on the State itself whether it will continue member of the Union. To deny this right, would be inconsistent with the principles on which all our political systems are founded; which is, the people have, in all cases, a right to determine how they will be governed."
The Rawle book (a minor judicial mind, by the way) was used for no more than one year (and probably less), and there's no evidence that the entire book was covered in classes or text.
I dare say MOST Americans in the north would have said in 1861 as Horace Greely did say "let our wayward sisters depart in peace".
This is exactly why lincoln went to so much trouble to start a war. Provoking the confederacy into "firing the first shot" was the only way he was going to sway northern sentiment enough to support his war.