Yikes, we'd be so screwed. Although from the reasons outlined in the book, I think it makes it very unlikely that actual aliens would come to the planet and look around before doing anything. They would probably just kill us from light years away.

You can't see sophons.

They can manipulate your retina to make you see whatever they want you to; UFOs, if they feel so inclined. But yes, not the particles themselves.

Anyway the shape is like a droplet.