You can ask your questions in German as well.

Just use WED (and MED) to design your levels or use advanced programs such as Blender (free!) and export them to WED, then setup entity and game logic related stuff via entity actions/ skills and so on.

Don't spend a another single minute with GED, it's way too buggy and just not worth the hassle. When you're somehow advanced and familiar with lite-C you can write your own (game specific) editor, too.