With a couple of small exceptions (like typo correction for misspelled option names), everything on the development plan checklist is existing functionality of the C++ driver that needs to be reimplemented before swift-driver can replace it.
Starting fairly recently, nightly trunk snapshots from swift.org have started shipping with swift-driver. Using it requires some tinkering with build settings though, or using it via SPM with the --use-integrated-swift-driver flag. There's some more info on that in this post and in the swift-driver README.
Not really. swift-driver is in pretty good shape currently, but there are still a few big unimplemented features like incremental builds, and a lot of small differences in behavior and test failures that need fixing before it can ship.
It's really up to you. Any changes to the C++ driver will eventually need to be ported to swift-driver, and swift-driver shouldn't include changes that break compatibility with the C++ version, but there's no requirement that you change both at the same time or restrict changes to one or the other.