You can play it for free offline by editing the command line it runs the game with and removing the "-demo" switch at the end. That will work with the Java version. Start the demo level, list the command line, paste it into a batch file. There is one temporary directory that is named after a GUID whose content you will need to copy while the game is running. The will turn the game into the full version, i.e. remove the demo counter that limits your playtime. Resource packs, launchers and so on should work, too. But you won't have a Realms ID.