Thank Tim, you pointed me exactly to where the issue was.
Here is where the confusion stems from for those suffering similar issues, and so maybe this can be made clearer in the future.
This checkbox is defaulting to 1, even though in all other cases the default is 0.
1 is baked tiling UVs, so clearly those DO overlap.
EDIT: Also, don’t use UV channel 0 for lightmaps. I thought you had to because it was showing up as the default on the other box, and I think because UDK used 0 as well.
But in UE4, if you put your textures on Channel 1, you then have to specify a texture coordinate node for every material that you place on the mesh, pain in the neck, not advised. 