And had Congress passed an amendment to end slavery what do you think the reaction would be, both in the states that seceded and those slave states that didn't?
It depends on when and how it was passed.
If a constitutional amendment had passed while southern states were in the union, then southern state votes would have been required to get the super majority needed.
And that means the successful amendment would have likely contained compromises - possibly compensation and a phaseout period.
The South had compromised several times previously on the slavery issue. The South wanted peace and the South loved the union created by Washington, Jefferson, Madison, Mason and the Lees.
The advantages of a constitutional amendment: it would have been peaceful and would not have required all the killings.
And if the peaceful constitutional amendment process had failed? The North would have still had the option of attacking, killing their political opponents, and destroying the South.