hdri, advance light map problem

Hello,

I have been using this scene provided in unreal 4.5 to light my scene.

everything works fine except that I find two issues:

1- i find the scene too noisy specially in the far distance and increasing the light map quality does not solve the problem.

2- once i build the light it says skybox lightmap overlapping ue4 so i did some research here and i followed the process described here:

attached you can see the result of the mesh once I finish the steps described in the post

I have no clue how to solve both I hope you guys can help me

Thanks