Everything is set up by channels. Like you mentioned your diffuse/albedo is channel 0. Depending on if you used multiple lightmap channels those will correspond with what channel you choose inside the engine to correspond with that mesh. So, the first lightmap channel that you make after your diffuse/albedo channel will be assigned to channel 1.
Looking at your UV’s for your second image on your lightmap channel i can already see the issue. You see where your UV’s and the points on the Map are in between the boxes. Essentially what that is doing is trying to calculate some kind of data. That data is computed in the lightmap and then assigned to your model. So what the light map is doing is computing vertex information on something that is essentially just a line and has no RGB value. When the lightmap artifacts like that it is the program trying to compute that RGB information along a numerical value between the two vertices.
In order to minimize this effect what you would need to do is to adjust your lightmap UV’s so that your outside lines fall in between the boxes on the grid. This minimizes the artifacting that you see. For more organic objects you would use a bump offset to move the values of your map slightly off of wherever you put your seams. I would not suggest this for walls and more hard edged objects.
Lastly hide your seams in places the player/viewer will never see. While this method reduces the effect it doesn’t eliminate it completely. In truth there is nothing to completely eliminate this problem there are ways to hide/minimize effects of the lightmap artifact. Also manually stitching together the geometry so that the map is one piece or very few will help as well. The lightmap has an easier time being calculated if it’s calculating for one set of uv’s as opposed to 20 like you would see if you tried to automatic map.
I hope this helps,