Much of this question wanders into "what if?" history in that any answer would be pure speculation. This subreddit asks members not to engage in those types of questions. I'll do my absolute best to give you something meaningful while respecting those rules.

There are usually three major events that historians talk about when the subject of possible Confederate victory is discussed. The Battle of Antietam, the Battle of Gettysburg, and the election of 1864.

The Battle of Antietam is cited because of two major reasons. First of all, it was a major attempt by the Confederacy to invade the North. It was generally agreed that for the United States to give up, victory would need to be won on their soil. Secondly, and more importantly, it was the closest the Confederacy ever came to being recognized by European powers (principally England and France). If the Confederates had won this battle, there is the possibility that one or both of those countries would have extended it recognition, which in turn (w/c)ould have meant armed intervention to ensure their sovereignty. However the Confederacy was not victorious, and instead it gave Lincoln the military "victory" (in quotes because it was really at best a draw, with the Union suffering ghastly casualties; however it DID end Lee's invasion) that he required in order to issue the Emancipation Proclamation.

The Battle of Gettysburg is often described as the closest the Confederacy ever came to victory. Most historians would disagree with that claim (if anything the concurrent Siege of Vicksburg was far more important and decisive; however it lacked the dramatic flair of Gettysburg). The reasons Gettysburg was considered important were quite similar to Antietam. Confederate victory in that battle MIGHT have meant European recognition. Whether that would have happened is irrelevant, since the Confederacy lost the battle. Most research into the matter reveals that Europe was cooling towards the Confederacy, and recognition might not have been forthcoming even in the event of a Confederate victory.

Finally is the election of 1864. As you state McClellan was running as a peace candidate, which at that time was starting to have some appeal to a war-weary North. However Lincoln was still popular with the average soldier. A combination of General Sherman's timely capture of Atlanta as well as extensive furloughing of soldiers (who Lincoln was still quite popular among) he was able to ensure a lopsided victory in the election.

As far as your question about McClellan's intentions of actually negotiating peace, it should be understood that his intention was never to surrender, but merely to negotiate a peace settlement that would spare both sides further bloodshed. It seems you understand that but just want to make sure. As for whether McClellan actually intended upon delivering what he promised, you could say as much as any politician ever does so.

The Confederacy actively sought British assistance in the Civil War, but the U.S. was able to keep European powers uninvolved. Britain itself was divided on the issue of supporting the Confederates; slavery had already been outlawed in Britain in 1861. If Britain had declared war on the U.S., events might have turned out very differently.

Pickett's Charge during the Battle of Gettysburg in 1863 is widely regarded as the high point of the Confederacy. If the U.S. had lost there, Lee's forces could have taken Harrisburg or even Philidalphia, PA, strongly hurting Union morale. history.state.gov has an excellent in-depth summary of politics of the Civil War, which I used for most of this information. You can find it here: https://history.state.gov/milestones/1861-1865/foreword