Lincoln did explore voluntary resettlement. You phrase it like he planned forced emigration - he did not. He also said that the black race was culturally inferior to whites - so what?
There is no constitutional right for states to secede.
That's because it is given by a higher power; God.
Therefore it trumps "constitution" as a source of rights. (God is actually the only source of rights. The Constitution does not give rights, it merely recognizes some of those that God has given. )
The Constitution says nothing about secession, because all that needed to be said on the subject was said 11 years earlier in the Declaration of Independence.