Yep, hard links only work within the same filesystem. You can have multiple drives in raid that form a single partition and use hard links within the array.
Basically yes. You use *arr to find releases and make a copy with proper naming and metadata when a download finishes. On its own, that would not be great as you would double the size of everything. Except you use hard links. Those are kind of like shortcuts, but both the shortcut and original are the same thing. Both point to the same data on disk. In fact, they’re indistinguishable from each other. If you delete one, the data remains as there is another link pointing to it. If you delete both, the data gets deleted. Basically they are free copies. You just have to make sure your file system supports them