Dark Edge on Fireplace in Dark Rooms

When I get shadows I can't explain my go to is to choose vertex, select whole mesh, then delete doubles, then select whole mesh again and go down to mesh, up to clean up and delete loose, including faces. Sometimes its as simple as a few errant verts or edges. 

I had a look at your mesh (Lod1Edit) and I have to ask what the bone shape thing is? I have never seen anything like this on a game mesh before.

I would use the fireplace as a guide and do as you said a complete rebuild. I have done that on many S2 conversions. A lot of them have odd things like the bone shape that the game won't see and is just extra polys, but mostly because you then get to uv map the fireplace yourself and will know if any problems arise where to go on the map to fix them. I often separate any parts of a mesh that look odd or that are throwing shadows using the P key, make sure the uv map is named uv_0 so you can easily rejoin the mesh when the editing is finished. Another trick is to look at the mesh using 'show weights' if you have vertex paint seeing a problem is difficult but seeing the mesh using the weight option means that some problems will show up regardless. Also switching faces to quads helps and viewing the mesh in solid and then going to Object Mode, this can show a shadow or a line that is easily missed when there is a lot of stuff going on on a mesh. Viewing in Rendered can also show problems. I also turn off the rig (click the eye beside it) when meshing as it can be annoying to see little dots that don't belong to mesh. Oh I get rid of the back plane the same way, they drive me insane 

When I removed the doubles and then ran clean up - delete loose, vertices, edges and faces I too got a dark shadow at the back, bottom right hand corner, its because the two vertices are separated there,  I highlighted both used Alt & M together and chose at center and the shadow vanished. Fireplaces can be a pain to make and get right. I hold off on the vertex paint until I am sure the mesh is ok, otherwise I can't see what I'm doing, I usually work in texture/shadeless mode so it's my own fault when I miss shadows, hence all the ways I use to find the problems 
I hope something here helps, sorry if I have repeated stuff you already know 

Sorry to hear it's still a problem. I wouldn't be surprised if it's not your mesh or the shape but the game lighting. I have had to delete the occluders on so many objects after an update that its almost become a ritual to check afterwards to see if anything is dodgy. Maybe store in a folder for another day (my fav trick when stuck).