1. Do you mean like a global variable? Those can be accessed through the system object on the event sheet editor.
2. I'm not quite sure what you mean here. Could you explain in a little more detail?
3. You can't create a group of variables. You could probably suggest it on the feature request board.
4. Use either the 'Compare Private Variable' event under your objects, or use the 'Compare' event under the System object which lets you compare any two things.
Hope that helps.