"Those congressmen of those states attempting to secede resigned from the Congress."
And where is the evidence that this was unlawful? Lincoln wasn't even most on the ballots in the South and he still won.
Lincoln was not on the ballots in Southern states because they did not allow it, now how is that lawful?
The Democratic Party split its own vote over the slave issue when Douglas would not give in to the slave block.
The South first seceded from the Democratic Party, then they attempted to secede from the Union and it is illegal because the Union is indivisible.
It would be no less illegal if the Northern States attempted to throw out the slave states from the Union.