1. In Disney-Speak, I take "late fall" to mean early Dec, when crowds are historically low(er) and they can get a feel for how Galaxy's Edge is running before the huge Christmas crowds descend, plus maybe boost attendance at an historically low(ish) time.
2. I'm willing to bet SW Half 2019 goes through Galaxy's Edge in some way - they already reworked the race theme and adding the new land would be a big marketing punch.
3. I don't think any of that needs to have anything to do with W&D, which can use the same course it's been using without issue.