I'm fine with the third possibility, that the slave family (who was not, I think, related to Isaac) was discovered by the Confederate patrol and returned to their owners in the original history. I think John Beckett did everything Sam did (got shot, went to Livvy's farm, saw the slaves, kept quiet about it), but it was Sam who devised the scheme of getting the watchmen drunk, and it was Al who figured out the route they should take to freedom (avoiding Confederate troops based on where the next couple of battles would occur). That is, I think, the point of diversion from the original history. Isaac was always going to be freed by Lincoln's Proclamation. I would imagine he would still feel like a "King" about that, without Sam's pep talk about how things were going to change in the future. I think the subtlety comes in because normally, Al has a wrap up regarding the people Sam is there to save, not some other character. But the point of the Underground Railroad (and the slave family was simply the last "passengers" to go through this particular "station," Livvy's farm) was that the slaves literally disappeared into the North. Ziggy would have no record to access to give an update. So if we're focused on the characters who get the update, we might think that Isaac is the focus of the Leap, or John and Livvy. But they're not. The family is. And we'll never know how they went on to change our history, or why GTFW wanted that particular wrong made right, because we don't know who they are.
I get what Al the Observer is saying about MLK existing because of Sam, but I don't think that fits in to the "affect little things" idea that permeated the series. If Sam saves Jackie, it's big but not earth shattering. If Sam saves MLK's existence - that'd be just a couple notches below Jesus Christ starting a new religion for world-changing power.