Thanks, you're completely correct, rights acquisition was the most difficult part!

The absolute hardest story in the anthology to get rights for was "Stars Don't Dream" by Chi Hui. It's a translation of a story that won an award in China, but Chi Hui doesn't speak English, and her contact info was extremely hard to obtain (I had to get help from the editor of Clarkesworld Magazine). We did the entire contract discussion via a combination of Google Translate and my very weak Mandarin I learned in college.

(I'm a huge Peter Watts fan, btw)