Can work experience overcome bad undergrad grades for admissions to masters programs in Germany?
The answer to all your questions depends on the university. There are only some rough standards which needs to be implemented by the universities.

If you consider computer science at my university the state dictated that one has to have a good bachelor to enroll in the master courses. Now there are two possibilities: Either you have a grade better than 2.5 (which is a B- in the European grading system and considered barely good) or you have to give reasons why your bachelor is weaker and then a committee has to decide whether you are allowed to enroll. But I know that other universities and even other departments at my university have other rules.