1. Instead of creating a linear extension in Sketch16, I grabbed one of the ends of the convert entities and drug it to extend it further. I did this on both ends to delete the tiny segments that were not a continuation of the primary surfaces.
2. Your problem was that these small surfaces would be enveloped in the offset/thicken, which SW can't handle. In your case, they should have been avoided entirely, but often you can do a variable shell to get around it. This is where you shell, but make that surface a thicker shell offset than the standard wall thickness.
3. I also used Shell instead of thicken, it has some more robust "checks" and also does some fudging for you. In this case, it worked!
The problem area. I removed these small segments at both ends:
COVER_3_cc.SLDPRT.zip 422.2 KB
with your help, we've managed to live to fight another day, and that's all that really matters, right?