The pre render shadows thing is because spring is tiling your map... the closest tile it could find for that bit of ground happend to have a small bit of notshadowed area on teh edge and hence when there all tiled next to each otehr they leave small lines...
I suggest rendering without the shadows and just letting spring do the work, or if you realy want them turn the compression way way down.
And Muff, you can access some of the mesas, but not all of them witout allterrain or airtransport.
(Well you could get blown up there by an explosion too, not probable tho.)