Hi, thanks a lot for building and testing.
Yes, the GTK AccessKit branch is already working on Windows. And it shouldn't be hard to make it work on macOS; I just have to write a bit of platform-specific code and figure out how to build on that platform.
I need to look into the first two bug reports. As for the third one, about stopping and restarting Orca, I think that's actually the same issue I'm working on right now.