The first channel is also for baked normal maps, if you try to use any other channel, you’ll get artifacts and seams.
UE4 might be adding it’s own channel for lightmaps for some reason, check your import settings. Also search to see if it might be a Blender specific issue UE4 users have, I know there’s been issues with Blender, FBX, and UE4 in the past, don’t know if it’s a 100% seamless process yet.
Yes, you can assign which channel UE4 uses for lightmaps in the static mesh editor.