A: RK was designed to work on Windows. Recommended is 'Windows 7', but older versions shouldn't cause any problmes. Hardware requirements are easily fulfilled if Warcraft III runs.
A: NO! Because we do not modify the original. All the modification happens in a temporal copy.
A: NORMALLY NOT! Because a backup is created before any of the patching is done. With this backup a recovery should be no problem. But to be 100% sure, make your own Warcraft 3 backup.
A: On Windows 7 most of your folders require special rights to write in them. Unfortunately we have to write files when patching is required, so we need those rights.
A: Make sure you watch replays by opening the replay files (rightclick->open or double click). If you watch it ingame via View Replay RK gets bypassed and has no effect!
A: It's clean :) If you don't trust me, download the source code (it's open source), read through it and compile it yourself. But the code ain't trivial anymore...
A: You have to delete it by hand. Removing ReplayKit.exe and the MPQ file should be enough. I don't think its worth the effort to delete the registry keys and the temporary files. But if you want to, feel free (info about all files and keys)!