That is odd, I haven't had UVs blow up like that when importing. Are you using ASE format and do you collapse the UV modifier before importing? Also, do you remember what the actual error is with the stairs? Invalid vertex coordinates, or something like that?
The first vertex ratio mesage is the error message I get when I forget to add a second uv channel. Are both channels assigned to the right thing in the editor (ie. Channel 1 > diffuse, channel 2 >lightmap)? Are your diffuse channel UVs the same as your lightmap channel UVs? If so, it may not be as easy to tell if your…