The Nail is a star, pretty clearly in the book (though I too thought it was the moon until long after I finished). I'm not at all sure how its connected to the Inchoroi, but it certainly seems that it must be, somehow.
Spin-off thought from that is that their ship didn't break down, but they attempted to use it an extinction level event orbital strike
I very much doubt this explanation. If you recall, the reason why the Inchoroi built the sranc was so that they'd exterminate the sentient life, but otherwise leave the world untouched. They make no villages or cities, they eat nothing but grubs when Nonmen/humans aren't around. The consult would not take such care creating those creatures if they wanted to ruin the world. You wouldn't nuke your Eden - there's no point of a promised land if its all radioactive and nuclear winter.
No, whatever caused the near disaster of their landing, including permanently scorching 500 miles of ground making it desolate for ever, turning the ground into broken glass and all that, it was not intentional. Or, at the very least, didn't work out as planned.
I would also guess that the Inchoroi didn't know Earwa was special until after they landed. Until after the ensuing centuries of wars against the Nonmen, or up until they saw magic.