I think that was being blocked by Swift features/libraries not being consistent across platforms, in that Swift for Linux/etc is missing stuff you'd get in macOS.
I don't see that changing any time soon. If Apple truly wanted Swift adoption to be cross platform, they have the resources to do it, but they didn't do it.
No, pretty sure that's all basically fixed in Swift 6 (Sep 2024), I think it's more just the immaturity of the C++ interop in being able to bridge Swift to complex C++ code.
That's a key feature of what Apple want Swift for (to gradually replace their C++ projects with Swift) but it's still pretty new. It'll take a while to mature.
I don't see that changing any time soon. If Apple truly wanted Swift adoption to be cross platform, they have the resources to do it, but they didn't do it.