This would all be solved if somebody just asked somebody at Mojang to say if this was a mod/plugin or not
What You Can Do
If you've bought the game, you may play around with it and modify it. We'd appreciate it if you didn't use this for griefing, though, and remember not to distribute the changed versions of our software. Basically, mods (or plugins, or tools) are cool (you can distribute those), hacked versions of the Minecraft client or server are not (you can't distribute those).
Any tools you write for the game from scratch belongs to you. Other than commercial use (unless specifically authorized by us in our brand and assets usage guidelines - for instance you are allowed to put ads on your YouTube videos containing Minecraft footage), you're free to do whatever you want with screenshots and videos of the game, but don't just rip art resources and pass them around, that's no fun. Plugins for the game also belong to you and you can do whatever you want with them, as long as you don't sell them for money. We reserve the final say regarding what constitutes a tool/plugin and what doesn't.