Short answer: Workout Writer supports current iOS/watchOS workflows, including Apple Watch and Garmin Connect. Android is not supported yet.
Not yet. Workout Writer currently focuses on iOS and watchOS.
See also: Device support matrix.
Minimum OS requirements can change over time as the app evolves.
The most reliable source is the current App Store listing, because it always reflects the latest supported iOS and watchOS versions for the current release.
No public date is available yet.
If Android support is important for your setup, please vote on the Feature Request board.