So, i just happened to see this post while searching for other lighting issues, and though it is old, nevertheless for the people that may find it through google:
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LightMAP UVs shouldnt overlap, since this may produce shadowing artifacts. Right now there is the option on import of the mesh to automatically generate Lightmap UVs (which is good enough most of the time).
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Slow Lighting build times are often related to not having a rather small LightMassImportanceVolume set. Just set it to enclose your area where the player can roam, and where you need the most detailed lighting. This shouldnt generally encompass your whole level
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Often enough with Foliage there is the problem that it doesnt work really good with static-lighting only. This produces really dark shadowing, whereas with a dynamic skylight/directional light everything looks perfect. IF you need static lighting only, you can open your static mesh, go to the lighting/lightmass setting and raise “diffuse boost” (which makes the mesh receive more lighting from static lights) to a value that is acceptable for you (start with 2.0 and go down as needed).
Edit: oh my, just saw the date on the post. Well, necropost ftw 
Cheers,