I don't believe that a Lee victory at Gettysburg would have been the cause of Lincoln's demise. There had been debacles before. If anything it would be a general loss of will by the norhern populace that had built up over the course of the war. If Lincoln was defeated and wanted to prosecute the war harder before he left office, I don't think he would have had much success getting his generals to move any faster. He had enough trouble before, and with the political nature of the senior officers of the army they would just have stalled until he was gone. Considering McClellans popularity in the army, he would probably have done quite a bit of manipulating himself considering his past record.
As for American freedoms being destroyed I think you are dead wrong. The freedoms I always hear being screamed about in the media are the "constitutional rights" of resident aliens and illegals who come here to gain all the benefits of being here without paying the price of admission or are here to cause harm to our nation. Don't constitutional rights belong to those who have the responsibilities of a citizen. You have no rights without responsibilities. I could be wrong here, so if I am, set me straight.

"The test of character is not 'hanging in' when you expect light at the end of the tunnel, but performance of duty and persistence of example when you know no light is coming."

-Admiral James B. Stockdale