For # 1, it is alwys advised to finish a Project in the version, in which it was started, but in general, PrE 9 Projects should do fine in PrE 10 - just not the other way around.
For the other two questions, I am a PC-only guy, so will have to let a Mac-user comment.
I plan to complete the project in 9 but I was concerned that downloading 10 will somehow conflict?
Installing version 10 on your computer will have absolutely no effect on version 9. They work in parallel.
And, no, Chroma Key has still not been added to the Mac version. The same effects and transitions that weren't include in Mac version 9 are omitted in version 10 also.
As I say in my books, however, the Videomerge effect does an excellent job doing the job usually done by Chroma Key.