We'll never really know if President Lincoln had made his proposal as a way of getting the idea of black sufferage started, with expanded sufferage to follow, or whether he really meant for it to be restricted to only some blacks. After all it was hardly universal in the Northern states in 1865, and perhaps President Lincoln saw Louisiana as the place to start. He must have thought that Governor Halm might be open to the idea, perhaps since of the confederate states Louisiana had the third largest percentage of free blacks prior to the war. But that is speculation. Instead he was assassinated and black sufferage for all was severely restricted by the southern state governments for another 100 years.
Your speculative excuse ignores the fact that the issue of black suffrage was already existant, and that in his very last speech Lincoln deliberately separated himself from those who wanted full rights for blacks. Contrary to what some revisionist lies imply, Lincoln did not invent the issue.
Instead he was assassinated and black sufferage for all was severely restricted by the southern state governments for another 100 years.
First, as you pointed out earlier, suffrage was not an executive decision, and Lincoln could not give blacks the vote anyway.
Second, your entire point is a non-sequitur, as it was Lincoln's political opponents within his own party who were the true advocates of black suffrage. If anything, it was easier for them to achieve black suffrage with him dead and not there to interfere with the rest of Congress by promoting his exclusionary and conditional alternatives to full rights.
Third, let us not forget the North. It was the Northern States that invented black codes and 'Jim Crow' type laws.