Lincoln had been calling for such an amendment for a while, but while it passed the senate, it couldn't pass in the house. In 1864, Lincoln pushed to have a call for the amendment added to the Republican platform. When he won the election and the Republicans gained seats in the house, it was able to pass. Lincoln signed the amendment, even though it was unnecessary and had never been done before to show his backing. Twenty-one states passed the amendment before Lincoln was assassinated.
Correct, signing the amendment was not needed and unconstitutional.
I’ll cede that Lincoln’s actions eventually freed the slaves ( a very good and necessary thing), but he did not do it through the Emancipation Proclamation.