Hi Mary!
For the first question, refer to my answer to James Clarke hashnode.com/ama/with-redux-cisteui6p005gzx53fstg…. The tl;dr version is that Dan's idea was to replace Flux stores with reducers. My contribution was to use reducer composition and selectors. And then we both came up with middleware and enhancers. That's a gross oversimplification, though, so you should read the full response :)
Regarding my role at Facebook, it's currently my fourth day as an employee, so for now I'm part of something called engineering bootcamp, which all new engineers participate in. After that I'll be joining the React team with Dan, which I'm super pumped about!