“In 1860 secessionists had neither mutual consent for at pleasure disunion nor material breeches of compact to justify secession from necessity.”
Historical perspective? Not on your side of the argument.
http://bonniebluepublishing.com/The%20Right%20of%20Secession.htm
Your link makes a lengthy case for a "right of secession", beginning by claiming Union states broke the Constitution's compact by enacting "Personal Liberty Laws" restricting returns of fugitive slaves.
Well, the first thing we should notice is that this does make secession, Lost Causers' frequent denials notwithstanding, "all about slavery".
The second is that Personal Liberty Laws were not a secession-worthy issue in 1850, after the Compromise of 1850 became law, because that Compromise shifted responsibility for Fugitive Slave enforcement from states to Federal government.
And the Supreme Court had already declared such laws unconstitutional.
So the Compromise of 1850 put Deep South Fire Eaters out of business for the next 10 years.
Third, those Deep South states which used Personal Liberty Laws as their excuse for secession had the least standing to complain because virtually none of their own slaves could escape through gauntlets of their own slave catchers into Upper South states, then through Border South States and even Border North states like Illinois, Indiana & New Jersey -- all of which had active slave-catcher patrols -- before reaching sanctuary in PLL states like Massachusetts or Michigan.
In fact, those Southern states which could lose the most fugitive slaves from Northern Personal Liberty Laws -- Border South states -- were least interested in using secession to stop it.
So claims that Personal Liberty Laws somehow justified secession in 1860 when they did not in the previous 10 years are simply specious.