1.) We can clearing the leaderboard/achievement/WebStorage data of all users for you manually, just send us an email if you’d like to do that
2.) You can start the game with different devices and play your game – however, the scoreboards will fill up really soon when you publish your game and the first 30 entries is mostly just a matter of minutes. If you publish a new game with Apple’s Game Center for example, you also start with an empty scoreboard.
3.) Tracking is different per platform. On PCs it’s the MAC address, on Android and iOS UDIDs (or what comes closest to that on each platform)