So, we have the progression of Lincoln's true sentiments, through all of his speeches -- yes, this is a fascinating thesis, one that deserves a closer look.
It sould not be considered a thesis at all, but a recognized fact by anyone who has studied Lincoln or the times. Yes, "Radicals" who openely advocated equality for all could be elected in isolated areas (Stephens in Pennsylvania and Sumner in Mass.), but in Illinois in the 1850s, or the country as a whole anyone who expressed the views of those Radical Republicans was commiting political suicide. (Think of Trent Lott calling for the Confederate Flag to be burned, or Rick Santorum calling for it to be raised over the PA Capitol.) You can't ask or expecte politicians to be too far out of phase whith their electorate. But you can ask them to lead, and Lincoln did.
By necessity, Lincoln walked a narrow path. Look to his private letters and even more to his actions. He first sought to keep the Union whole. But he did it in the context of ending slavery and making the nation true to the Declaration Principles. When ran for office in 1860, not a single intelligent political observer would have predicted the end of slavery in another 5 years. Not 10% of the voters in the country would have voted for that. But it happend, and 90% of the people supported ending slavery because of Lincoln's leadership.